This adds a port to Linux on the IBM Z platform (a.k.a s390x). It only
supports the 64-bit ABI. Most functionality is working and all the tests
pass with the exception of the coredump tests*.
Unwinding is only supported if DWARF unwind information is present.
libunwind can't currently make use of the backchain (if present).
The getcontext/setcontext functions only preserve/restore a subset of
registers. Currently this only consists of callee-saved registers and
some parameter registers.
Vector registers and access registers are not saved (and aren't callee-
saved) by getcontext and cannot currently be modified. They will however
be restored unmodified after resuming a context from a signal handler.
There is no special libunwind support for setjmp, the functionality is
emulated using glibc (I think all the ports do this for modern Linux
kernels).
* Unwinding on s390x requires floating point register access which the
coredump library doesn't currently support.
