PowerPC SPE targets don't have `spe` target feature enabled

[beetrees]

I tried this code when targeting any of the PowerPC SPE targets such as powerpc-unknown-linux-muslspe (compiler explorer):

pub fn add(x: f32, y: f32) -> f32 {
    x + y
}

I expected to see this happen: The assembly to use SPE instructions as this is an SPE target.

Instead, this happened: The assembly uses regular PowerPC FPU floating point instructions, as none of the three powerpc-*spe targets actually have the spe target feature enabled. According to the Debian PowerPCSPEPort wiki page, PowerPC SPE lacks the standard PowerPC FPU, and the Signal Processing Engine (SPE) Programming Environments Manual doesn't list fadds (which the targets currently compile the x + y to) as an SPE instruction, so the status quo is almost certainly incorrect.

cc @BKPepe as target maintainer of powerpc-unknown-linux-muslspe. Also cc @glaubitz who added powerpc-unknown-linux-gnuspe in #48484 and @BaoshanPang who added powerpc-wrs-vxworks-spe in #61946, as neither of those two targets have any target maintainers.

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rustc --version --verbose:

rustc 1.87.0-nightly (aa8f0fd71 2025-03-23)
binary: rustc
commit-hash: aa8f0fd7163a2f23aa958faed30c9c2b77b934a5
commit-date: 2025-03-23
host: x86_64-unknown-linux-gnu
release: 1.87.0-nightly
LLVM version: 20.1.1

[beetrees]

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

[RalfJung]

Hm yeah, that's odd... we would have expected +spe to be listed here, right?

rust/compiler/rustc_target/src/spec/targets/powerpc_wrs_vxworks_spe.rs

Line 28 in b660382

features: "+secure-plt,+msync".into(),

[beetrees]

Yes. The spe feature will also be enabled when using a SPE target CPU such as e500 (none of the SPE targets currently specify a default target CPU so they all default to the generic PowerPC one), but being explicit would definitely be better.