package/valgrind: hide comment when arch is not supported

Currently, the comment that "valgrind needs shared libs" is not hidden
when the architecture dependencies are not met, which can confuse some
users (as recently seen on IRC).

Fix that by introducing the traditional _ARCH_SUPPORTS option, and have
the comment and the symbol depend on that.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Brandon Maier <brandon.maier@rockwellcollins.com>
Cc: Jérôme Pouiller <jezz@sysmic.org>
Cc: Peter Seiderer <ps.report@gmx.net>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Reviewed--by: Pedro Ribeiro <pedrib@gmail.com>
Tested-by: Pedro Ribeiro <pedrib@gmail.com>
[Arnout: put _ARCH_SUPPORTS at the top of the file]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>