Enable building libomp.so for 32-bit x86. This is done by selectively
enabling the functions that save and restore MXCSR, since access to this
register requires SSE support.

Note that you may run into other issues with OpenMP on i386, since this
*not* yet supported upstream, and certainly not extensively tested.

PR: 236062, 236582
MFC after: 1 month
X-MFC-With: r344779