Change most internal uses of time to __clock_gettime.

As for gettimeofday, time will be implemented based on clock_gettime
on all platforms and internal code should use clock_gettime
directly.  In addition to removing a layer of indirection, this will
allow us to remove the PLT-bypass gunk for gettimeofday.

The changed code always assumes __clock_gettime (CLOCK_REALTIME)
or __clock_gettime (CLOCK_REALTIME_COARSE) (for Linux case) cannot
fail, using the same rationale for gettimeofday change.  And internal
helper was added (time_now).

Checked on x86_64-linux-gnu, i686-linux-gnu, powerpc64le-linux-gnu,
powerpc64-linux-gnu, and powerpc-linux-gnu.

Reviewed-by: Lukasz Majewski <lukma@denx.de>