[libc-coord] Add new ABI '__memcmpeq()' to libc

[Joseph Myers]

On Fri, 17 Sep 2021, Richard Biener via Gcc wrote:

> when the libc version targeted does not provide __memcmpeq?  Or would
> glibc through <string.h> magically communicate the availability of the new ABI
> without actually declaring the function?
> (I'm not sure whether a GCC build-time decision via configure is the
> very best idea)

I was supposing a build-time decision (using GCC_GLIBC_VERSION_GTE_IFELSE 
to know if the glibc version on the target definitely has this function).  
But if we add a header declaration, you could check for __memcmpeq being 
declared (and so cover arbitrary C libraries, not just glibc, and avoid 
issues of needing to disable this logic for freestanding compilations, 
which would otherwise be an issue if a glibc-target toolchain is used for 
a freestanding kernel compilation).  The case of people calling 
__builtin_memcmp (or declaring memcmp themselves) without string.h 
included probably isn't one it's important to optimize.

-- 
Joseph S. Myers
joseph@codesourcery.com