string/memchr.c: Merge from gnulib

string/memchr.c: Merge from gnulib

Merge most of the gnulib implementation of memchr. The changes that
remain are:

 - copyright header
 - bp-sym.h removed
 - reg_char removed
 - allow MEMCHR to be redefined
 - non-conforming whitespace changes

The merged code fixes a number of -Wundef warnings and also introduces
an optimized algorithm. I haven't detected any performance difference
in the new code which I believe is down to the quite specific
circumstances required to hit it. However the new code is approximately
half the size of the old code on AArch64 (which uses generic memchr).

ChangeLog:

2014-07-04  Will Newton  <will.newton@linaro.org>

	* string/memchr.c: Merge from gnulib.
	[_LIBC]: Remove conditionals.
	(__ptr_t): Remove define.
	(LONG_MAX_32_BITS): Likewise.
	(LONG_MAX): Likewise.
	(MEMCHR): Use ANSI prototype and optimize algorithm.