Makefile: fix location of listing produced by "make subdir/foo.s"

When I invoke "make block-sha1/sha1.s", 'make' runs $(CC) -S without specifying where it should put its output and the output ends up in ./sha1.s. Confusing. Add an -o option to the .s rule to fix this. We were already doing that for most compiler invocations but had forgotten it for the assembler listings. Signed-off-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>