Allow retrieving x86 TSC frequency/flags from CPUID

Allow retrieving x86 TSC frequency/flags from CPUID

This adds additional x86 specific CPUID checks for flags needed for
determining whether the Time-Stamp Counter (TSC) is usable on a given system,
as well as a helper function to retrieve the TSC frequency from CPUID.

This is intended for a future patch that will utilize the TSC to lower the
overhead of timing instrumentation.

In passing, always make pg_cpuid_subleaf reset the variables used for its
result, to avoid accidentally using stale results if __get_cpuid_count errors
out and the caller doesn't check for it.

Author: Lukas Fittl <lukas@fittl.com>
Author: David Geier <geidav.pg@gmail.com>
Author: Andres Freund <andres@anarazel.de>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: David Geier <geidav.pg@gmail.com>
Reviewed-by: John Naylor <john.naylor@postgresql.org>
Reviewed-by: Jakub Wartak <jakub.wartak@enterprisedb.com> (in an earlier version)
Discussion: https://www.postgresql.org/message-id/flat/20200612232810.f46nbqkdhbutzqdg%40alap3.anarazel.de