Fix build of C mempcpy and stpcpy.

Fix build of C mempcpy and stpcpy.

This patch fixes the build of C mempcpy and stpcpy by disabling the
redirection to __mempcpy and __stpcpy asm names if
NO_MEMPCPY_STPCPY_REDIRECT is defined, and defining that macro in the
relevant source files.

Tested for powerpc32 that the build is fixed.

	* include/string.h [NO_MEMPCPY_STPCPY_REDIRECT] (mempcpy): Do not
	redeclare with asm name.
	[NO_MEMPCPY_STPCPY_REDIRECT] (stpcpy): Likewise.
	* string/mempcpy.c (NO_MEMPCPY_STPCPY_REDIRECT): Define before
	including <string.h>.
	* string/stpcpy.c (NO_MEMPCPY_STPCPY_REDIRECT): Likewise.
	* sysdeps/powerpc/powerpc32/power4/multiarch/mempcpy.c
	[!NOT_IN_libc] (NO_MEMPCPY_STPCPY_REDIRECT): Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/mempcpy.c
	[!NOT_IN_libc] (NO_MEMPCPY_STPCPY_REDIRECT): Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/stpcpy.c
	[SHARED && !NOT_IN_libc] (NO_MEMPCPY_STPCPY_REDIRECT): Likewise.