[AArch64] Make wfxt a full Extension (#90987)

Before #90987 WFxT did not have an AEK_WFXT, so it was assumed to be an
FMV-only extension. However it also had a SubtargetFeature. This commit
combines the two. This fixes an issue where -mattr=+wfxt was ignored,
but has the side effect of allowing +wfxt as an option to -march.