[SystemZ] Add z13 vector facility and MC support

This patch adds support for the z13 processor type and its vector facility,
and adds MC support for all new instructions provided by that facilily.

Apart from defining the new instructions, the main changes are:

- Adding VR128, VR64 and VR32 register classes.
- Making FP64 a subclass of VR64 and FP32 a subclass of VR32.
- Adding a D(V,B) addressing mode for scatter/gather operations
- Adding 1-, 2-, and 3-bit immediate operands for some 4-bit fields.
  Until now all immediate operands have been the same width as the
  underlying field (hence the assert->return change in decode[SU]ImmOperand).

In addition, sys::getHostCPUName is extended to detect running natively
on a z13 machine.

Based on a patch by Richard Sandiford.

llvm-svn: 236520