aarch64: PR target/105157 Increase number of cores TARGET_CPU_DEFAULT can encode

aarch64: PR target/105157 Increase number of cores TARGET_CPU_DEFAULT can encode

This addresses the compile-time increase seen in the PR target/105157. This was
being caused by selecting the wrong core tuning, as when we added the latest
AArch64 the TARGET_CPU_generic tuning was pushed beyond the 0x3f mask we used
to encode both target cpu and attributes into TARGET_CPU_DEFAULT.

gcc/ChangeLog:

	PR target/105157
	* config.gcc: Shift ext_mask by TARGET_CPU_NBITS.
	* config/aarch64/aarch64.h (TARGET_CPU_NBITS): New macro.
	(TARGET_CPU_MASK): Likewise.
	(TARGET_CPU_DEFAULT): Use TARGET_CPU_NBITS.
	* config/aarch64/aarch64.cc (aarch64_get_tune_cpu): Use TARGET_CPU_MASK.
	(aarch64_get_arch): Likewise.
	(aarch64_override_options): Use TARGET_CPU_NBITS.