[X86] Support -mlong-double-80

Add an option group for all of the -mlong-double-* options and make
-mlong-double-80 restore the default long double behavior for X86.  The
motivations are that GNU accepts the -mlong-double-80 option and that complex
Makefiles often need a way of undoing earlier options. Prior to this commit, if
one chooses 64-bit or 128-bit long double for X86, there is no way to undo that
choice and restore the 80-bit behavior.

Differential Revision: https://reviews.llvm.org/D66055

llvm-svn: 369183