Externalize the _bfd_set_gp_value function

This change adds an external-visible wrapper for the _bfd_set_gp_value
function.  This is a prerequisite for some gdb patches that better
handle powerpc64le relocations against ".TOC.".

	* bfd.c (bfd_set_gp_value): New externally visible wrapper
	for _bfd_set_gp_value.
	* bfd-in2.h: Regenerate.