ArmPkg/CompilerIntrinsicsLib: make the default memset() weak

ArmPkg/CompilerIntrinsicsLib: make the default memset() weak

The ARM compiler intrinsics library defines __aeabi_memset() and
memset() in the same object, which means that both will be pulled
in if either is referenced.

The IntrinsicLib in CryptoPkg defines its own, preferred memset(),
which may clash with our memset(). So make our version weak.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>