Run fewer math tests (#20860)

Move some to other when they are really just testing a libc method. Mark others
as wasm2js-only when they seem like they test bit operations or other things
that wasm2js needs coverage for (in multiple opt levels).

Remove test_getgep as it was just testing trivial struct operations that have
lots of other coverage (and it doesn't even test LLVM getelementptr directly).

Author: kripken