[clang] [llvm] [ARM] Armv8-R does not require fp64 or neon. (PR #88287)

[Chris Copeland via cfe-commits]

chrisnc wrote:

@davemgreen which change? Specifying `-mcpu=cortex-r52` will behave the same way as before. The original manual for the R52 provided for a no-neon sp-only variant, and they exist in the wild, and this lets "architecture-generic" builds automatically support both.

One example where this comes up is in the Rust project, which recently gained armv8r support, but due to the over-spec'ing of the base armv8r in LLVM, it requires `-feature` additions in the default target feature list to build the `core` crate in a way that will support lesser r52's by default. Then, when users specify cortex-r52 as their target cpu, they are still left with the `-feature` additions overriding what the r52 should enable by default. https://github.com/rust-lang/rust/pull/123159 This change will allow the feature flags and target CPUs to interact in a more predictable way as compared to other Cortex-R and -M CPUs.

https://github.com/llvm/llvm-project/pull/88287