Make.config: Use a different method to detect --build-id support.
Some versions of ld accept /dev/null as a valid input file. Others will not. The latter versions will give false negatives for --buil-id support, which breaks linking of Pintos userspace programs. So use a different method, that I hope will be more reliable, to detect --build-id support. Reported by Joseph Huang <firstname.lastname@example.org>.