Remove "powerpc64le" and "mipsel" target_arch
Remove "powerpc64le" and "mipsel" target_arch

Currently the `mipsel-unknown-linux-gnu` target doesn't actually set the
`target_arch` value to `mipsel` but it rather uses `mips`. Alternatively the
`powerpc64le` target does indeed set the `target_arch` as `powerpc64le`,
causing a bit of inconsistency between theset two.

As these are just the same instance of one instruction set, let's use
`target_endian` to switch between them and only set the `target_arch` as one
value. This should cut down on the number of `#[cfg]` annotations necessary and
all around be a little more ergonomic.

rust-lang/reference,rust-lang/reference