[Haiku] Support __float128 for x86 and x86_64

This patch addresses a compilation error with clang when
running in Haiku being unable to compile code using
float128 (throws compilation error such as 'float128 is
not supported on this target').

Patch by kallisti5 (Alexander von Gluck IV)

Differential Revision: https://reviews.llvm.org/D54901

llvm-svn: 348368