fix(libc): memset inclusion to libc makefiles

Fix [1] is introducing another problem in that memset is added twice to
the libc makefile when OVERRIDE_LIBC=1 (the C and asm implementations).

Correct by adding memset.c when OVERRIDE_LIBC=0 and memset.S when
OVERRIDE_LIBC=1.

[1] https://review.trustedfirmware.org/c/TF-A/trusted-firmware-a/+/26091

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: Ie4b7e04880d4cd871e7b51cd8ff5bddcf8d0308c
2 files changed