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

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>