Auto merge of #136146 - RalfJung:x86-abi, r=workingjubilee

Explicitly choose x86 softfloat/hardfloat ABI

Part of rust-lang/rust#135408:
Instead of choosing this based on the target features listed in the target spec, make that choice explicit.
All built-in targets are being updated here; custom (JSON-defined) x86 (32bit and 64bit) softfloat targets need to explicitly set `rustc-abi` to `x86-softfloat`.