[AArch64] Add Linux hwcap strings for some extensions

This patch adds feature strings for some of the extensions. This string is what is read from /proc/cpuinfo on Linux systems
and used during -march=native detection.

The strings are taken from the kernel source tree at:
https://github.com/torvalds/linux/blob/master/arch/arm64/kernel/cpuinfo.c#L45


	* config/aarch64/aarch64-option-extensions.def (sb): Add feature
	string.
	(ssbs): Likewise.
	(sve2): Likewise.
	(sve2-sm4): Likewise.
	(sveaes): Likewise.
	(svesha3): Likewise.
	(svebitperm): Likewise.

From-SVN: r275331