x86_64 ABI: Two bug fixes.

1. Return of _Complex long double used wrong type.

2. va_arg of types passed in two SSE registers didn't account for
extra space in register save area.

Down to 18 failures on gcc/compat/x86_64. Combined 32/64 results are:
--
		=== gcc Summary ===

# of expected passes		1292
# of unexpected failures	34
# of unsupported tests		2
--

llvm-svn: 64880