Makefile: move ".SUFFIXES" rule to shared.mak

Makefile: move ".SUFFIXES" rule to shared.mak

This was added in 30248886ce8 (Makefile: disable default implicit
rules, 2010-01-26), let's move it to the top of "shared.mak" so it'll
apply to all our Makefiles.

This doesn't benefit the main Makefile at all, since it already had
the rule, but since we're including shared.mak in other Makefiles
starts to benefit them. E.g. running the 'man" target is now faster:

    $ git -c hyperfine.hook.setup= hyperfine -L rev HEAD~1,HEAD~0 -s 'make -C Documentation man' 'make -C Documentation -j1 man'
    Benchmark 1: make -C Documentation -j1 man' in 'HEAD~1
      Time (mean ± σ):     121.7 ms ±   8.8 ms    [User: 105.8 ms, System: 18.6 ms]
      Range (min … max):   112.8 ms … 148.4 ms    26 runs

    Benchmark 2: make -C Documentation -j1 man' in 'HEAD~0
      Time (mean ± σ):      97.5 ms ±   8.0 ms    [User: 80.1 ms, System: 20.1 ms]
      Range (min … max):    89.8 ms … 111.8 ms    32 runs

    Summary
      'make -C Documentation -j1 man' in 'HEAD~0' ran
        1.25 ± 0.14 times faster than 'make -C Documentation -j1 man' in 'HEAD~1'

The reason for that can be seen when comparing that run with
"--debug=a". Without this change making a target like "git-status.1"
will cause "make" to consider not only "git-status.txt", but
"git-status.txt.o", as well as numerous other implicit suffixes such
as ".c", ".cc", ".cpp" etc. See [1] for a more detailed before/after
example.

So this is causing us to omit a bunch of work we didn't need to
do. For making "git-status.1" the "--debug=a" output is reduced from
~140k lines to ~6k.

1. https://lore.kernel.org/git/220222.86bkyz875k.gmgdl@evledraar.gmail.com/

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