This patch addresses varargs processing for small complex types under
the 64-bit PowerPC ELF ABI.

The ABI requires that the real and imaginary parts of a complex argument
each occupy their own doubleword.  Arguments smaller than 8 bytes are
right-adjusted within the doubleword.

Clang expects EmitVAARG() to return a pointer to a structure in which
the real and imaginary parts are packed adjacently in memory.  To accomplish
this, we generate code to load the code appropriately from the varargs
location and pack the values into a temporary variable in the form Clang
expects, returning a pointer to that structure.

The test case demonstrates correct code generation for all "small" complex
types on PPC64:  int, short, char, and float.