Adjust JIT target triple on OS X to match the current architecture.
Adjust JIT target triple on OS X to match the current architecture.

For OS X builds, we generate one version of config.h but then build for
multiple architectures.  This means that the LLVM_HOSTTRIPLE setting may have
the wrong architecture.  Adjust it dynamically to match the current
architecture.  <rdar://problem/12715470>

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@169405 91177308-0d34-0410-b5e6-96231b3b80d8

dslab-epfl/asap,GPUOpen-Drivers/llvm,apple/swift-llvm,dslab-epfl/asap,chubbymaggie/asap,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,chubbymaggie/asap,dslab-epfl/asap,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,chubbymaggie/asap,chubbymaggie/asap,dslab-epfl/asap,dslab-epfl/asap,llvm-mirror/llvm,apple/swift-llvm,chubbymaggie/asap,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,chubbymaggie/asap,apple/swift-llvm,llvm-mirror/llvm,dslab-epfl/asap,llvm-mirror/llvm,dslab-epfl/asap,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm