Refer to C23 in place of C2X in glibc

WG14 decided to use the name C23 as the informal name of the next
revision of the C standard (notwithstanding the publication date in
2024).  Update references to C2X in glibc to use the C23 name.

This is intended to update everything *except* where it involves
renaming files (the changes involving renaming tests are intended to
be done separately).  In the case of the _ISOC2X_SOURCE feature test
macro - the only user-visible interface involved - support for that
macro is kept for backwards compatibility, while adding
_ISOC23_SOURCE.

Tested for x86_64.

79 files changed, 300 insertions, 278 deletions

diff --git a/NEWS b/NEWS
index 39a55f5767..2d8eaffc58 100644
--- a/NEWS
+++ b/NEWS
@@ -14,6 +14,13 @@ Major new features:
   in order to support unsigned __int128 and/or unsigned _BitInt(N) operands
   with arbitrary precisions when supported by the target.
 
+* The GNU C Library now supports a feature test macro _ISOC23_SOURCE to
+  enable features from the ISO C23 standard.  Only some features from
+  this standard are supported by the GNU C Library.  The older name
+  _ISOC2X_SOURCE is still supported.  Features from C23 are also enabled
+  by _GNU_SOURCE, or by compiling with the GCC options -std=c23,
+  -std=gnu23, -std=c2x or -std=gnu2x.
+
 Deprecated and removed features, and other changes affecting compatibility:
 
   [Add deprecations, removals and changes affecting compatibility here]
diff --git a/bits/fenv.h b/bits/fenv.h
index 9f19295d65..aa0c549d4a 100644
--- a/bits/fenv.h
+++ b/bits/fenv.h
@@ -54,7 +54,7 @@ fenv_t;
 /* If the default argument is used we use this value.  */
 #define FE_DFL_ENV	((const fenv_t *) -1l)
 
-#if __GLIBC_USE (IEC_60559_BFP_EXT_C2X)
+#if __GLIBC_USE (IEC_60559_BFP_EXT_C23)
 /* Type representing floating-point control modes.  */
 typedef unsigned int femode_t;
 
diff --git a/bits/libc-header-start.h b/bits/libc-header-start.h
index d841f5d8cf..4ab1304b0d 100644
--- a/bits/libc-header-start.h
+++ b/bits/libc-header-start.h
@@ -44,23 +44,23 @@
 
 /* ISO/IEC TS 18661-1:2014 defines the __STDC_WANT_IEC_60559_BFP_EXT__
    macro.  Most but not all symbols enabled by that macro in TS
-   18661-1 are enabled unconditionally in C2X.  In C2X, the symbols in
+   18661-1 are enabled unconditionally in C23.  In C23, the symbols in
    Annex F still require a new feature test macro
-   __STDC_WANT_IEC_60559_EXT__ instead (C2X does not define
+   __STDC_WANT_IEC_60559_EXT__ instead (C23 does not define
    __STDC_WANT_IEC_60559_BFP_EXT__), while a few features from TS
-   18661-1 are not included in C2X (and thus should depend on
-   __STDC_WANT_IEC_60559_BFP_EXT__ even when C2X features are
+   18661-1 are not included in C23 (and thus should depend on
+   __STDC_WANT_IEC_60559_BFP_EXT__ even when C23 features are
    enabled).
 
    __GLIBC_USE (IEC_60559_BFP_EXT) controls those features from TS
-   18661-1 not included in C2X.
+   18661-1 not included in C23.
 
-   __GLIBC_USE (IEC_60559_BFP_EXT_C2X) controls those features from TS
-   18661-1 that are also included in C2X (with no feature test macro
-   required in C2X).
+   __GLIBC_USE (IEC_60559_BFP_EXT_C23) controls those features from TS
+   18661-1 that are also included in C23 (with no feature test macro
+   required in C23).
 
    __GLIBC_USE (IEC_60559_EXT) controls those features from TS 18661-1
-   that are included in C2X but conditional on
+   that are included in C23 but conditional on
    __STDC_WANT_IEC_60559_EXT__.  (There are currently no features
    conditional on __STDC_WANT_IEC_60559_EXT__ that are not in TS
    18661-1.)  */
@@ -70,11 +70,11 @@
 #else
 # define __GLIBC_USE_IEC_60559_BFP_EXT 0
 #endif
-#undef __GLIBC_USE_IEC_60559_BFP_EXT_C2X
-#if __GLIBC_USE (IEC_60559_BFP_EXT) || __GLIBC_USE (ISOC2X)
-# define __GLIBC_USE_IEC_60559_BFP_EXT_C2X 1
+#undef __GLIBC_USE_IEC_60559_BFP_EXT_C23
+#if __GLIBC_USE (IEC_60559_BFP_EXT) || __GLIBC_USE (ISOC23)
+# define __GLIBC_USE_IEC_60559_BFP_EXT_C23 1
 #else
-# define __GLIBC_USE_IEC_60559_BFP_EXT_C2X 0
+# define __GLIBC_USE_IEC_60559_BFP_EXT_C23 0
 #endif
 #undef __GLIBC_USE_IEC_60559_EXT
 #if __GLIBC_USE (IEC_60559_BFP_EXT) || defined __STDC_WANT_IEC_60559_EXT__
@@ -86,18 +86,18 @@
 /* ISO/IEC TS 18661-4:2015 defines the
    __STDC_WANT_IEC_60559_FUNCS_EXT__ macro.  Other than the reduction
    functions, the symbols from this TS are enabled unconditionally in
-   C2X.  */
+   C23.  */
 #undef __GLIBC_USE_IEC_60559_FUNCS_EXT
 #if defined __USE_GNU || defined __STDC_WANT_IEC_60559_FUNCS_EXT__
 # define __GLIBC_USE_IEC_60559_FUNCS_EXT 1
 #else
 # define __GLIBC_USE_IEC_60559_FUNCS_EXT 0
 #endif
-#undef __GLIBC_USE_IEC_60559_FUNCS_EXT_C2X
-#if __GLIBC_USE (IEC_60559_FUNCS_EXT) || __GLIBC_USE (ISOC2X)
-# define __GLIBC_USE_IEC_60559_FUNCS_EXT_C2X 1
+#undef __GLIBC_USE_IEC_60559_FUNCS_EXT_C23
+#if __GLIBC_USE (IEC_60559_FUNCS_EXT) || __GLIBC_USE (ISOC23)
+# define __GLIBC_USE_IEC_60559_FUNCS_EXT_C23 1
 #else
-# define __GLIBC_USE_IEC_60559_FUNCS_EXT_C2X 0
+# define __GLIBC_USE_IEC_60559_FUNCS_EXT_C23 0
 #endif
 
 /* ISO/IEC TS 18661-3:2015 defines the
diff --git a/inc