string: Replace = with += in CFLAGS-xxx.c

Replace = with += in CFLAGS-xxx.c to allow Makefile under sysdeps to
define CFLAGS-xx.c.

	* string/Makefile (CFLAGS-inl-tester.c): Replace = with +=.
	(CFLAGS-noinl-tester.c): Likewise.
	(CFLAGS-tst-strlen.c): Likewise.
	(CFLAGS-stratcliff.c): Likewise.
	(CFLAGS-test-ffs.c): Likewise.
	(CFLAGS-tst-inlcall.c): Likewise.
	(CFLAGS-tst-xbzero-opt.c): Likewise.
	(CFLAGS-memcpy.c): Likewise.
	(CFLAGS-wordcopy.c): Likewise.