rt-tests: enable on MIPS uClibc again

We have fixes in the internal toolchain; an external toolchain is most
likely a buildroot-built toolchain anyway, so it will also have the
fix. But we still keep a comment to warn the user for potentially-broken
external toolchains.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>