LLVM_DEFAULT_TARGET_TRIPLE was defined in config.h and llvm-config.h,
which was causing warnings when compiling it for Fuchsia. Also made
some other cleanups in llvm-config.h, particularly removed i386 and arm
ifdefs.

There are still some other warnings not fixed in this CL
('using namespace' in headers), but these don't look Fuchsia-specific.

Bug: 881334
Change-Id: Iaf54e622e9d31553268afe960d6330dcb5920321
Reviewed-on: https://swiftshader-review.googlesource.com/c/21689Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Sergey Ulanov <sergeyu@chromium.org>