oeqa/selftest/rust: skip on all MIPS platforms

oeqa/selftest/rust: skip on all MIPS platforms

As per "The rustc book"[1], mips*-unknown-linux-* are "tier 3" targets:

  Tier 3 targets are those which the Rust codebase has support for, but
  which the Rust project does not build or test automatically, so they
  may or may not work.

We already skip qemumips in this selftest, but we're now also seeing
failures with qemumips64 so refactor the test to skip all machines where
the architecture is mips or mips64.

[1] https://doc.rust-lang.org/nightly/rustc/platform-support.html

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>