Auto merge of #36369 - uweigand:s390x, r=alexcrichton

Auto merge of #36369 - uweigand:s390x, r=alexcrichton

Add s390x support

This adds support for building the Rust compiler and standard
library for s390x-linux, allowing a full cross-bootstrap sequence
to complete.  This includes:

- Makefile/configure changes to allow native s390x builds
- Full Rust compiler support for the s390x C ABI
  (only the non-vector ABI is supported at this point)
- Port of the standard library to s390x
- Update the liblibc submodule to a version including s390x support
- Testsuite fixes to allow clean "make check" on s390x

Caveats:

- Resets base cpu to "z10" to bring support in sync with the default
  behaviour of other compilers on the platforms.  (Usually, upstream
  supports all older processors; a distribution build may then chose
  to require a more recent base version.)  (Also, using zEC12 causes
  failures in the valgrind tests since valgrind doesn't fully support
  this CPU yet.)

- z13 vector ABI is not yet supported.  To ensure compatible code
  generation, the -vector feature is passed to LLVM.  Note that this
  means that even when compiling for z13, no vector instructions
  will be used.  In the future, support for the vector ABI should be
  added (this will require common code support for different ABIs
  that need different data_layout strings on the same platform).

- Two test cases are (temporarily) ignored on s390x to allow passing
  the test suite.  The underlying issues still need to be fixed:
  * debuginfo/simd.rs fails because of incorrect debug information.
    This seems to be a LLVM bug (also seen with C code).
  * run-pass/union/union-basic.rs simply seems to be incorrect for
    all big-endian platforms.

Signed-off-by: Ulrich Weigand <ulrich.weigand@de.ibm.com>