[X86][PowerPC] Support -mlong-double-128

[X86][PowerPC] Support -mlong-double-128

This patch makes the driver option -mlong-double-128 available for X86
and PowerPC. The CC1 option -mlong-double-128 is available on all targets
for users to test on unsupported targets.

On PowerPC, -mlong-double-128 uses the IBM extended double format
because we don't support -mabi=ieeelongdouble yet (D64283).

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D64277

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@365866 91177308-0d34-0410-b5e6-96231b3b80d8