mips: Replace magic number '4' by a symbolic constant (number of GP register parameters).
mips: Replace magic number '4' by a symbolic constant (number of GP register parameters).

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