Support ARM64.

This hides the rdtsc assembler instruction behind a macro and implements on aarch64 by using ARM system clock counter instruction.

Two SMP test programs fail as they use the intel cpuid instruction (to find the currently executing processor ? The EAX register is not set to 0x1 before calling cpuid, however... #ifdef's in the code seem to indicate that they don't work on 64bit intel, in which case they should just be removed.