vfs: implement utimensat() and futimens()

Add utimensat() and futimens() which can update file timestamps with
nanosecond precision.

Implementation to handle AT_SYMLINK_NOFOLLOW flag is enabled by default
as namei currently does not follow symbolic links.

Signed-off-by: Jaspal Singh Dhillon <jaspal.iiith@gmail.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
tests: add test for utimensat()

Add tests for utimensat() and add it to build.mk

Signed-off-by: Jaspal Singh Dhillon <jaspal.iiith@gmail.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>