This patch enables general varargs support for the 64-bit PPC SVR4 ABI.

Most of the pieces for this were already in place, but a proper EmitVAArg
is needed for aggregates and complex numbers to be handled.  Although the
va_list for 64-bit PowerPC SVR4 consists of GPRs 3 through 10 together with
the overflow portion of the parameter save area, we can treat va_list as
pointing to contiguous memory for all parameters, since the back end forces
the parameter GPRs to memory for varargs functions.

There is no need at this time to model parameters and return values beyond
what the DefaultABIInfo provides.

llvm-svn: 165143