Makefile: move $(comma), $(empty) and $(space) to shared.mak

Makefile: move $(comma), $(empty) and $(space) to shared.mak

Move these variables over to the shared.mak, we'll make use of them in
a subsequent commit.

Note that there's reason for these to be "simply expanded variables",
i.e. to use ":=" assignments instead of lazily expanded "="
assignments. We could use "=", but let's leave this as-is for now for
ease of review.

See 425ca6710b2 (Makefile: allow combining UBSan with other
sanitizers, 2017-07-15) for the commit that introduced these.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>