support negative number, so we can set all bits of a debug mask with -1
support negative number, so we can set all bits of a debug mask with -1

[r8596]

libfirm/libfirm,jonashaag/libfirm,jonashaag/libfirm,davidgiven/libfirm,jonashaag/libfirm,MatzeB/libfirm,jonashaag/libfirm,davidgiven/libfirm,jonashaag/libfirm,8l/libfirm,MatzeB/libfirm,davidgiven/libfirm,davidgiven/libfirm,MatzeB/libfirm,killbug2004/libfirm,8l/libfirm,killbug2004/libfirm,davidgiven/libfirm,8l/libfirm,libfirm/libfirm,8l/libfirm,MatzeB/libfirm,8l/libfirm,jonashaag/libfirm,killbug2004/libfirm,libfirm/libfirm,killbug2004/libfirm,davidgiven/libfirm,jonashaag/libfirm,killbug2004/libfirm,MatzeB/libfirm,davidgiven/libfirm,MatzeB/libfirm,libfirm/libfirm,8l/libfirm,MatzeB/libfirm,killbug2004/libfirm,killbug2004/libfirm,libfirm/libfirm,8l/libfirm