libc: Fall back to rdtsc when using pvclock and rdtscp is not available

[markj]

Summary

Now that kvmclock can optionally export timekeeping info to userspace
even on systems without rdtscp, we should fall back to using rdtsc when
necessary. This mimics pvclock_read_time_info() in the kernel.

Tested by: Shrikanth R Kamath <[e-mail]>

[kib]

I am curious, what is the hw/sw configuration where RDTSCP is not available? I believe the instruction itself was added at least with Nehalem (2009).

[markj]

In D38341#871021, @kib wrote:

I am curious, what is the hw/sw configuration where RDTSCP is not available? I believe the instruction itself was added at least with Nehalem (2009).

By default, the machine emulated by qemu-system-x86_64 is missing many "newer" capabilities. I did not ask for the exact configuration, but in my testing, the default -machine and -cpu parameters give you a system without AMDID_RDTSCP. So, this diff is mostly useful in cases where one does not control the (poorly chosen) hypervisor configuration.