[PATCH] Add -mlong-double-64 flag

[Taiju Tsuiki]

>>! In D6260#8, @rnk wrote:
> This needs an IRGen test. We should produce the LLVM 'double' type instead of the x86_fp80 type.

OK, I added a test to the CL.

I think another things to consider is LongDoubleAlign.
LongDoubleAlign is 32 on x86_32 and 64 on x86_64, should I adjust it in TargetInfo::adjust too for each supported arch?

http://reviews.llvm.org/D6260