Add clang_darwin_embedded platform for embedded projects

This should build a separate set of embedded runtime libraries, supporting the
option product:

    {PIC, static} x { Hard-float, soft-float }

The emphasis is on ARM platforms (Cortex-M4F, Cortex-M3, Cortex-M0) but X86
variants are also built where possible.

rdar://problem/10817205

llvm-svn: 194873