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.

src/liballoc_system/lib.rs

@@ -29,10 +29,8 @@ extern crate libc;
 #[cfg(all(any(target_arch = "x86",
               target_arch = "arm",
               target_arch = "mips",
-              target_arch = "mipsel",
               target_arch = "powerpc",
-              target_arch = "powerpc64",
-              target_arch = "powerpc64le")))]
+              target_arch = "powerpc64")))]
 const MIN_ALIGN: usize = 8;
 #[cfg(all(any(target_arch = "x86_64",
               target_arch = "aarch64")))]