Add NativeClient operating system support.

This patch adds support of NativeClient (*-*-nacl) OS support to LLVM.
It's already supported in autoconf/config.sub.

The motivation for this change is to start upstreaming PNaCl work. The
whole set of patches include llvm backends (i686, x86_64, ARM),
llvm-gcc (probably, would not be upstreamed because it's deprecated)
and clang (the work has been just started, the amount of changes is
going to be low and the most of the work is expected to be done close
to the mainline).

llvm-svn: 138005