Add non-SSE wrapper for __kmp_{load,store}_mxcsr

Summary:
To be able to successfully build OpenMP on FreeBSD/i386, which still
uses i486 as its default processor, I had to provide wrappers for the
`__kmp_load_mxcsr` and `__kmp_store_mxcsr` functions.

If the compiler signals that SSE is not available, loading and storing
mxcsr does not make sense anway, so in that case the inline functions
are empty.  This gives the minimum amount of code churn.

See also https://svnweb.freebsd.org/changeset/base/345283

Reviewers: emaste, jlpeyton, Hahnfeld

Reviewed By: jlpeyton

Subscribers: hfinkel, krytarowski, jdoerfert, openmp-commits, llvm-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D60916

llvm-svn: 360062