*-symbol-check: Don't hard-code nm executable

Helpful if your nm executable has a prefix based on the
architecture, for example.

Signed-off-by: Heiko Becker <heirecka@exherbo.org>
Cc: Timo Gurr <timo.gurr@gmail.com>
[Eric: v2: rebase and add Meson support]
Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>