aarch64: Add ISA requirements to some SVE/SME md comments

aarch64: Add ISA requirements to some SVE/SME md comments

The SVE and SME md files are divided into sections, with each
section often starting with a comment that lists the associated
mnemonics.  These lists usually include the base architecture
requirement in parentheses, if the base requirement is greater
than the baseline for the file.  This patch tries to be more
consistent about when we do that for the recently added SVE2p1
and SME2p1 extensions.

gcc/
	* config/aarch64/aarch64-sme.md: In the section comments, add the
	architecture requirements alongside some mnemonics.
	* config/aarch64/aarch64-sve2.md: Likewise.