Makefile: learn to generate listings for targets requiring special flags

Makefile: learn to generate listings for targets requiring special flags

'make git.s' to debug code generation of main() fails because
git.c makes use of preprocessor symbols such as GIT_VERSION that
are not set.  make does not generate code listings for
builtin_help.c, exec_cmd.c, builtin-init-db.c, config.c, http.c,
or http-walker.c either, for the same reason.

So pass the flags used to generate each .o file when generating
the corresponding assembler listing.

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>