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>

include/arch/arc/v2/asm_inline_gcc.h

@@ -21,6 +21,12 @@ extern "C" {
 #include <stdint.h>
 #include <stddef.h>
 
+/**
+ *  @brief read timestamp register (CPU frequency)
+ */
+extern uint64_t _tsc_read(void);
+
+
 /* Implementation of sys_io.h's documented functions */
 
 static ALWAYS_INLINE