arc: add _tsc_read for 64-bit timestamp

This implementation of _tsc_read returns a 64-bit value that
is derived from the 64-bit tick count multiplied by hwcycles per tick,
and then it adds the current value from the 32-bit timer.
This produces a 64-bit time. There is a bunch of math here, which
could be avoided if the CPU is built with Real-Time-Clock option.
EM Starter Kit SOCs don't have this. I don't think Arduino 101 does
either.

See ZEP-1559

Change-Id: I9f846d170246556ac40fe2f45809e457c6375d8c
Signed-off-by: Chuck Jordan <cjordan@synopsys.com>