linux: add support for clock_getres through syscall

Commit 49497bdb689 added support for clock_gettime through syscall, because one
of the C++ libraries uses it this way. Turns out clock_getres can also be
reached the same way.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>