arm64: Kconfig: Enable usage of optimized memset/memcpy/memmove

This patch enables the use of the optimized memset(), memmove() &
memcpy() versions recently added on ARM64.

Please note that these optimized functions are now only enabled for
recent GCC versions (>= 9.4), as earlier GCC versions throw these
errors:

aarch64-linux-ar: warning: arch/arm/lib/memset-arm64.o: unsupported GNU_PROPERTY_TYPE (5) type: 0xc0000000
...

Signed-off-by: Stefan Roese <sr@denx.de>
[trini: Make this default to off as it causes problems on some platforms still]
Signed-off-by: Tom Rini <trini@konsulko.com>