libiberty, gcc: Add memrchr to libiberty and use it [PR119283].

libiberty, gcc: Add memrchr to libiberty and use it [PR119283].

This adds an implementation of memrchr to libiberty and arranges
to configure gcc to use it, if the host does not have it.

	PR cobol/119283

gcc/ChangeLog:

	* config.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Check for host memrchr.

include/ChangeLog:

	* libiberty.h (memrchr): New.

libiberty/ChangeLog:

	* Makefile.in: Add memrchr build rules.
	* config.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Check for memrchr.
	* functions.texi: Document memrchr.
	* memrchr.c: New file.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>