odd endianness toolchains for crosstool

["Jason A. Donenfeld" <Jason@zx2c4.com>]

Hey Arnd,

I'm again experimenting with switching to your crosstool toolchains for
WireGuard's CI. I've hit a few snags in the process:

- For powerpc, gcc needs to be built with `--enable-secureplt
  --with-long-double-64` in order for musl to run.
- Need powerpc64le compiler (-mabi=elfv2).
- Need mipsel compiler.
- Need aarch64_be compiler.
- Need armeb compiler.
- Need mips64el compiler.

While the existing compilers can all produce code for the wrong endian,
they hit trouble when trying to link against libgcc. So generally a
separate full toolchain is supplied for the less common endians.

I have had success with arm, arm64, mips, x86_64, i386, m68k. If you're
up for adding the above compilers to the collection, I'd be able to
complete the transition, and then look into adding a few more
architectures.

Jason