Makefile: regenerate assembler listings when asked

Makefile: regenerate assembler listings when asked

'make var.s' fails to regenerate an assembler listing if var.c
has not changed but a header it includes has:

	$ make var.s
	    CC var.s
	$ touch cache.h
	$ make var.s
	$

The corresponding problem for 'make var.o' does not occur because
the Makefile lists dependencies for each .o target explicitly;
analogous dependency rules for the .s targets are not present.
Rather than add some, it seems better to force 'make' to always
regenerate assembler listings, since the assembler listing
targets are only invoked when specifically requested on the make
command line.

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>