Change x86mmx -> x86_mmx in LangRef.rst

The correct name of the type in LLVM assembly is "x86_mmx".  Also remove
the reST label "t_x86mmx" because it was unused anyway.

Patch by Manuel Jacob!

Differential Revision: http://llvm-reviews.chandlerc.com/D2955

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202929 91177308-0d34-0410-b5e6-96231b3b80d8