[RFC] Implement memcchr

[Yuri Gribov]

Hi all.

Recent FreeBSD have a nice complement for standard memchr function -
memcchr (https://www.freebsd.org/cgi/man.cgi?query=memcchr). It e.g.
allows user to efficiently check whether buffer is zeroed out. Would
it make sense to add something like this to Glibc as well (yes, I can
volunteer)?

Here's a motivational GCC bug:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69908

Adhemerval expressed his concerns that the function may not be that
widespread and useful.


-Y