Update clang support on recent Haiku

[ Copied from https://llvm.org/bugs/show_bug.cgi?id=26404 ]

clang support on Haiku is lagging a bit, and missing on x86_64.

This patch updates support for x86 and add support for x86_64. It should
apply directly to trunk and it's harmless in the sense that it only
affects Haiku.

Reviewers: rnk, rsmith

Patch by Jérôme Duval

Differential Revision: http://reviews.llvm.org/D16797

llvm-svn: 269201