[libc] Exclude few unused bits from x86 state for Windows

Windows fenv_t does not include the MXCSR register and
the unused bits at the end of the x87 status. So we
exclude them in our struct definitions to make it
easy to read/write the state. getEnv and setEnv
were also excluded to avoid using MXCSR, but a
forthcoming patch will handle these functions.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D106386