[KMSAN] Enable on SystemZ

Enable -fsanitize=kernel-memory support in Clang.

The x86_64 ABI requires that shadow_origin_ptr_t must be returned via a
register pair, and the s390x ABI requires that it must be returned via
memory pointed to by a hidden parameter. Normally Clang takes care of
the ABI, but the sanitizers run long after it, so unfortunately they
have to duplicate the ABI logic.

Therefore add a special case for SystemZ and manually emit the
s390x-ABI-compliant calling sequences. Since it's only 2 architectures,
do not create a VarArgHelper-like abstraction layer.

The kernel functions are compiled with the "packed-stack" and
"use-soft-float" attributes. For the "packed-stack" functions, it's not
correct for copyRegSaveArea() to copy 160 bytes of shadow and origins,
since the save area is dynamically sized. Things are greatly simplified
by the fact that the vararg "use-soft-float" functions use precisely
56 bytes in order to save the argument registers to where va_arg() can
find them.

Make copyRegSaveArea() copy only 56 bytes in the "use-soft-float" case.
The "packed-stack" && !"use-soft-float" case has no practical uses at
the moment, so leave it for the future.

Add tests.

Reviewed By: eugenis

Differential Revision: https://reviews.llvm.org/D148596