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
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D1308