Double-check pow implementation on MIPS

[vks]

In Rand, we use essentially the same implementation as in in num_traits, but it fails to return the correct result on the MIPS architecture: rust-random/rand#752

It seems likely that the num implementation is also affected by this issue.

[cuviper]

Does it give the same wrong answer on debug and release builds? Have you stepped through to see where it goes wrong? Either way, if this only fails on MIPS, it seems more likely to be a codegen issue to report to the compiler.