library/std/build.rs: "powerpc64le" is not a target_arch

The target_arch of `powerpc64le` is `powerpc64`, so
`powerpc64le` can be removed from a match arm in build.rs
related to f16.

You can check available `target_arch`:s with:

    $ rustc +nightly -Zunstable-options --print all-target-specs-json \
            | grep powerpc | grep arch | sort | uniq
        "arch": "powerpc",
        "arch": "powerpc64",