Floating point flags access

[joehendrix]

The C standard includes rules about when floating point functions are allowed to modify floating point state, and when one may read floating point status registers. See FENV_ACCESS and FENV_ROUND in http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2346.pdf

We should inspect these rules, and ensure we generate legal LLVM when we access floating point status/control registers.