fix(libc): add memcpy_s source file to libc_asm mk

Change [1] introduced the memcpy_s function and added the source file to
lib/libc/libc.mk but omitted to update lib/libc/libc_asm.mk
Arm platforms (and platforms from one partner) use OVERRIDE_LIBC=1
option as a platform default hence consume libc_asm.mk
To prevent this confusion to happen again, create libc_common.mk for the
common set of C files to build.
libc_common.mk is included by both libc.mk and libc_asm.mk
The latter adds asm implementations of libc functions.

[1] https://review.trustedfirmware.org/c/TF-A/trusted-firmware-a/+/21450

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: Ibd36ea5c561e35f268048ccbefc8f47485f64bdf
3 files changed