Rollup merge of #157137 - taiki-e:powerpcspe, r=RalfJung

rustc_target: Use +spe for powerpcspe targets

LLVM does not infer this from the target name and `abi: "spe"` is just for cfg.  To actually generate the correct instructions for these targets (without `-C target-cpu`), we need to pass `+spe` to LLVM.

Fixes rust-lang/rust#138960
See also rust-lang/rust#157085
Related rust-lang/rust#137860

cc @RalfJung
cc @BKPepe ([powerpc-unknown-linux-muslspe target maintainer](https://doc.rust-lang.org/nightly/rustc/platform-support/powerpc-unknown-linux-muslspe.html#target-maintainers))
cc @glaubitz (who added powerpc-unknown-linux-gnuspe in rust-lang/rust#48484)
cc @biabbas @hax0kartik ([*-wrs-vxworks target maintainers](https://doc.rust-lang.org/nightly/rustc/platform-support/vxworks.html#target-maintainers))

@rustbot label +O-PowerPC +A-target-feature

Author: JonathanBrouwer