Correctly Load Mixed FP-GP Variadic Arguments for x86-64.

According to the x86-64 ABI, structures with both floating point and
integer members are split between floating-point and general purpose
registers, and consecutive 32-bit floats can be packed into a single
floating point register.

In the case of variadic functions these are stored to memory and the position
recorded in the va_list. This was already correctly implemented in
llvm.va_start.

The problem is that the code in clang for implementing va_arg was reading
floating point registers from the wrong location.

Patch by Thomas Jablin.

Fixes PR20018.