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

[Florian Weimer]

* Joseph Myers:

> 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.

The header-less case looks relevant to C++ and other language front
ends, though.  So a GCC_GLIBC_VERSION_GTE_IFELSE check could still make
sense for them.

(Dropping libc-coord.)

Thanks,
Florian