Fix cycleclock.h for gcc/ARM.

Currently there are tests for ARMV3 and ARMV6 in cycleclock.h which are not
defined using gcc on ARM. Since there is also a cast to the unknown type
int64 I assume that the ARM code has not been tested. Therefore this patch
replaces the checks for ARMV3 and ARMV6 by checks for __ARM_ARCH. Also, the
cast to int64 is fixed by casting to int64_t.