libtool: Discard "-mllvm $arg" options when linking.

libtool: Discard "-mllvm $arg" options when linking.

Clang accepts options of the form "-mllvm $arg", and passes the
argument as an option to LLVM.  These options caused problems
for Libtool when linking; in some cases, the -mllvm option is
passed through but the corresponding argument is dropped.  (See
for example http://llvm.org/bugs/show_bug.cgi?id=14716.)

This patch resolves the issue by explicitly matching -mllvm and
taking an argument.  Since Clang never uses LLVM for linking,
the matched "-mllvm $arg" option is irrelevant for the link
step, and we can simply discard it once we've recognized it.

Co-authored-by: Johannes Obermayr <johannesobermayr@gmx.de>
Copyright-paperwork-exempt: Yes