Makefile: add an explicit rule for building assembly output

Makefile: add an explicit rule for building assembly output

In the kernel we have a rule for *.c -> *.s files exactly because
it's nice to be able to easily say "ok, what does that generate".

Here's a patch to add such a rule to git too, in case anybody is
interested. It makes it much simpler to just do

	make sha1_file.s

and look at the compiler-generated output that way, rather than having to
fire up gdb on the resulting binary.

(Add -fverbose-asm or something if you want to, it can make the result
even more readable)

[jc: add *.s to .gitignore]

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