Let Clang invoke CloudABI's linker.

Now that CloudABI's target information and header search logic for Clang
has been submitted, the only thing that remains to be done is adding
support for CloudABI's linker.

CloudABI uses Binutils ld, although there is some work to use lld
instead. This means that this code is largely based on what we use on
FreeBSD. There are some exceptions, however:

- Only static linking is performed. CloudABI does not support any
  dynamically linked executables.
- CloudABI uses compiler-rt, libc++ and libc++abi unconditionally. Link
  in these libraries instead of using libgcc_s, libstdc++, etc.
- We must ensure that the .eh_frame_hdr is present to make C++
  exceptions work properly.


Differential Revision:	http://reviews.llvm.org/D8250

llvm-svn: 233269