aarch64: Fix narrowing warning in driver-aarch64.cc [PR118603]

aarch64: Fix narrowing warning in driver-aarch64.cc [PR118603]

Since the AARCH64_CORE defines in aarch64-cores.def all use -1 for
the variant, it is just easier to add the cast to unsigned in the usage
in driver-aarch64.cc.

Build and tested on aarch64-linux-gnu.

gcc/ChangeLog:

	PR target/118603
	* config/aarch64/driver-aarch64.cc (aarch64_cpu_data): Add cast to unsigned
	to VARIANT of the define AARCH64_CORE.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>