fiber: use clock_monotonic64 instead of __rdtscp

clock_gettime(CLOCK_MONOTONIC) is implemented via the RDTSCP instruction
on x86 an has the following advantages over the raw instruction:

* It checks for RDTSCP availability in CPUID.
  If RDTSCP is not supported, it switches to RDTSC.
* Linux guarantee that clock is monotonic, hence, the CPU miss
  detection is not needed.
* It works on ARM.

As for disadvantage, this function is about 2x slower compared to a
single RDTSCP instruction. Performance degradation measured by the
fiber switch benchmark [1] is about 3-7% for num_fibers == 10-1000.

Closes #5869

[1] https://github.com/tarantool/tarantool/issues/2694#issuecomment-546381304

NO_DOC=bugfix
NO_TEST=<Tested in test/app/fiber.test.lua>