meson: recognize "sunos" as the system name for Solaris

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Acked-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
(cherry picked from commit d8a9420f)
Minor conflicts resolved by Dylan Baker