Add SystemZ support

This patch then adds all the usual platform-specific pieces for SystemZ:
driver support, basic target info, register names and constraints,
ABI info and vararg support.  It also adds new tests to verify pre-defined
macros and inline asm, and updates a test for the minimum alignment change.

This version of the patch incorporates feedback from reviews by
Eric Christopher and John McCall.  Thanks to all reviewers!

Patch by Richard Sandiford.