[X86] Add -malign-double support
The -malign-double flag causes i64 and f64 types to have alignment 8
instead of 4. On x86-64, the behavior of -malign-double is enabled by default.
Rebases and cleans phosek's work here: http://reviews.llvm.org/D12860
Patch by Sean Klein
Subscribers: rnk, jfb, dschuff, phosek
Differential Revision: http://reviews.llvm.org/D19734