Auto merge of #36028 - japaric:s390x, r=alexcrichton

Auto merge of #36028 - japaric:s390x, r=alexcrichton

initial support for s390x

A new target, `s390x-unknown-linux-gnu`, has been added to the compiler
and can be used to build no_core/no_std Rust programs.

Known limitations:

- librustc_trans/cabi_s390x.rs is missing. This means no support for
  `extern "C" fn`.
- No support for this arch in libc. This means std can't be cross
  compiled for this target.

r? @alexcrichton

This time I couldn't test running a binary cross compiled to this target under QEMU because the qemu-s390x that ships with Ubuntu 16.04 SIGABRTs with every s390x binary I run it with.

Change in binary size of `librustc_llvm.so`:

Without this commit (stage1): 41895736 bytes
With this commit (stage1): 42899016 bytes

~2.4% increase