rustc_target: Use +spe for powerpcspe targets

[taiki-e]

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 #138960
See also #157085
Related #137860

cc @RalfJung
cc @BKPepe (powerpc-unknown-linux-muslspe target maintainer)
cc @glaubitz (who added powerpc-unknown-linux-gnuspe in #48484)
cc @biabbas @hax0kartik (*-wrs-vxworks target maintainers)

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

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[hax0kartik]

LGTM, but please give me some time to test this out for VxWorks(its Sat here, so 2-3 days)

View changes since this review

[mu001999]

Waiting for the test for VxWorks

@rustbot blocked

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[glaubitz]

Interesting, I thought what the spec file is currently doing would be enough to enable SPE support by passing -mspe and telling LLVM to use the powerpc-unknown-linux-gnuspe target.

Thanks for fixing this!

[RalfJung]

@glaubitz it only passes -mspe to the linker, which does nothing for LLVM codegen.
And apparently the LLVM target name on its own does nothing.

[glaubitz]

@glaubitz it only passes -mspe to the linker, which does nothing for LLVM codegen. And apparently the LLVM target name on its own does nothing.

Yeah, makes sense. Good catch in any case!