Rollup merge of #137595 - folkertdev:remove-simd-pow-powi, r=RalfJung

remove `simd_fpow` and `simd_fpowi`

Discussed in https://github.com/rust-lang/rust/issues/137555

These functions are not exposed from `std::intrinsics::simd`, and not used anywhere outside of the compiler. They also don't lower to particularly good code at least on the major ISAs (I checked x86_64, aarch64, s390x, powerpc), where the vector is just spilled to the stack and scalar functions are used for the actual logic.

r? `@RalfJung`

compiler/rustc_span/src/symbol.rs

@@ -1877,8 +1877,6 @@ symbols! {
         simd_fma,
         simd_fmax,
         simd_fmin,
-        simd_fpow,
-        simd_fpowi,
         simd_fsin,
         simd_fsqrt,
         simd_gather,