C2x makes static_assert and thread_local into keywords, removing the
definitions as macros in assert.h and threads.h. Thus, disable those
macros in those glibc headers for C2x.
The disabling is done based on a combination of language version and
__GNUC_PREREQ, *not* based on __GLIBC_USE (ISOC2X), on the principle
that users of the header (when requesting C11 or later APIs - not
assert.h for C99 and older API versions) should always have the names
static_assert or thread_local available after inclusion of the header,
whether as a keyword or as a macro. Thus, when using a compiler
without the keywords (whether an older compiler, possibly in C2x mode,
or _GNU_SOURCE with any compiler but in an older language mode, for
example) the macros should be defined, even when C2x APIs have been
requested. The __GNUC_PREREQ conditionals here may well need updating
with the versions of other compilers that gained support for these
keywords in C2x mode.
Tested for x86_64.
