[clang] [llvm] [AArch64] Fix feature flags dependecies (PR #90612)

[Paul Walker via cfe-commits]

paulwalker-arm wrote:

@momchil-velikov's commentary applies globally and is not specific to FPMR. Which is to say, Arm switched a while back from "all system register need to be protected by their feature flag" to "only protect system registers where there is a need".  The rational is that we see it as being unnecessarily burdensome to asm writers to force them to use a feature flag in order to use the pretty printed version of an instruction they can already emit (this is especially true when dynamic feature detection is used, rather than wanting to explicitly say the feature must be present) .  We've no direct plans to revisit all previously implemented system registers unless there's a specific need.

https://github.com/llvm/llvm-project/pull/90612