amd64_cconv: Always reserve a multiple of 8 bytes on the stack.
amd64_cconv: Always reserve a multiple of 8 bytes on the stack.

This is relevant for long doubles, which are 80 bits long, but take up
128 bits on the stack and in memory according to the AMD64 ABI.

jonashaag/libfirm,MatzeB/libfirm,MatzeB/libfirm,libfirm/libfirm,jonashaag/libfirm,libfirm/libfirm,MatzeB/libfirm,libfirm/libfirm,MatzeB/libfirm,libfirm/libfirm,libfirm/libfirm,jonashaag/libfirm,jonashaag/libfirm,MatzeB/libfirm,jonashaag/libfirm,jonashaag/libfirm,MatzeB/libfirm,MatzeB/libfirm,jonashaag/libfirm