Remove powerpcspe support (1/2)

[mchoo7]

I made many mistakes in #1908 . As per @chmeeedalf 's request, I created a new pull request that only removes powerpcspe.

This is the first stage of two-stage removal process of powerpcspe.

1. Remove powerpc/powerpcspe from build system and isolated folders (e.g. libc/powerpcspe). Remove files that are exclusively used by powerpc/powerpcspe as well.
2. Remove powerpc/powerpcspe code under preprocessor macros (e.g. SPE).

Support for MPC85XXSPE is removed.