Provide a proper fenv implementation

[erikdesjardins]

See #106 (comment)

src/math/fenv.rs currently contains a dummy implementation of feraiseexcept, fegetround and friends. It should be properly implemented.

[erikdesjardins]

Aside: I feel like these tricks from fma ought to be able to replace some of the fenv usage in fmaf, but I don't understand the subtleties.

[hanna-kruppe]

Floating point environment access is currently not supported by LLVM (see e.g. https://lists.llvm.org/pipermail/llvm-dev/2018-May/123529.html) so it won't be possible to provide fenv access in any code that is compiled by LLVM. (This affects the musl C code too if it's compiled by Clang.)

[tgross35]

I am going to close this in favor of a more meta discussion at rust-lang/compiler-builtins#842.