MFC r275759:

Pull in r221170 from upstream clang trunk (by Roman Divacky):

  Implement vaarg lowering for ppc32. Lowering of scalars and
  aggregates is supported. Complex numbers are not.

This adds va_args support for PowerPC (32 bit) to clang.

Reviewed by:	jhibbits
Differential Revision: https://reviews.freebsd.org/D1308