syscalls/getrusage04: Try guess timer granularity.

This patch adds timer granularity detection for the testcase.

The granularity of getrusage timers depends on CONFIG_HZ kernel
parameter. There is unfortunately no API to get getrusage timers
resolution but there are linux specific POSIX timers that have exactly
same granularity. These are now used to detect the expected timers
resolution.

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
syscalls/getrusage04: Link rt through LDLIBS instead of LDFLAGS.

Fixes this build error on Ubuntu introduced with commit
07f4e5432dd3e7a60bc0235f13200640c82d8f89.

testcases/kernel/syscalls/getrusage/getrusage04.c:186: undefined reference to `clock_getres'

Signed-off-by: Vinson Lee <vlee@twitter.com>