[libc] Support the string conversion methods on the GPU

This patch enables us to use the existing `libc` support for string
conversion functions on the GPU. This required setting the `fenv_t` and
long double configuration. As far as I am aware, long doubles are
converted to doubles on the GPU and the floating point environment is
just an `uint32_t`.

This code is still untested as we are still working out how to run the
unit tests on the GPU.

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D149306