[PATCH] [x32] Introduce -mx32 flag enforcing x32 mode for other triplets. By Pavel Chupin (@pavel.v.chupin).

-mx32 flag setup target environment to GNUX32 and can be used for
other 32/64-bit triplets (i386-unknown-linux, x86_64-unknown-linux) to
turn on x32 mode. Compatible with GCC -mx32 flag.

Differential Revision: http://reviews.llvm.org/D4470