aarch64 timer is available to userspace via arch register.
clang's __builtin_readcyclecounter is wrong for aarch64 (reads the PMU
cycle counter instead of the archi-timer register), so we don't use it.
my_rdtsc unit-test on AWS m6g shows:
frequency: 121830845
resolution: 1
overhead: 1
This counter is not strictly increasing, but it is non-decreasing.
On clang, use __builtin_readcyclecounter() when available.
Hinted by Sergey Vojtovich. (This may lead to runtime failure
on ARM systems. The hardware should be available on ARMv8 (AArch64),
but access to it may require special privileges.)
We remove support for the proprietary Sun Microsystems compiler,
and rely on clang or the __GNUC__ assembler syntax instead.
For now, we retain support for IA-64 (Itanium) and 32-bit SPARC,
even though those platforms are likely no longer widely used.
We remove support for clock_gettime(CLOCK_SGI_CYCLE),
because Silicon Graphics ceased supporting IRIX in December 2013.
This was the only cycle timer interface available for MIPS.
On PowerPC, we rely on the GCC 4.8 __builtin_ppc_get_timebase()
(or clang __builtin_readcyclecounter()), which should be equivalent
to the old assembler code on both 64-bit and 32-bit targets.
