aarch64: Fix up TME deprecation warning

This is a follow-up patch to Richard E's r17-2498-g17f084306c68c4 (#186
on the Forge).  As Alice pointed out in her review:
https://gcc.gnu.org/pipermail/gcc-patches/2026-July/724391.html
the original patch missed things like +memtag+tme due to the string
parsing approach.  This instead just inspects the parsed feature flags
from aarch64_parse_extension to determine whether to warn in the target
attribute/pragma case.

+tme on the command line is handled as per Richard's original patch (no
change there).

gcc/ChangeLog:

	* config/aarch64/aarch64.cc (aarch64_handle_attr_isa_flags):
	Warn if +tme is enabled by the target attribute/pragma, relying
	on the feature flags from aarch64_parse_extension.
	(aarch64_process_target_attr): Drop +tme warning based on custom
	string parsing.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/deprecate-tme.c: New test.