pgsql: pg_test_timing: Also test RDTSC[P] timing, report time source, T

pg_test_timing: Also test RDTSC[P] timing, report time source, TSC frequency

This adds support to pg_test_timing for the different timing sources added by
294520c4448.

Author: Lukas Fittl <lukas(at)fittl(dot)com>
Author: David Geier <geidav(dot)pg(at)gmail(dot)com>
Reviewed-by: Andres Freund <andres(at)anarazel(dot)de>
Reviewed-by: David Geier <geidav(dot)pg(at)gmail(dot)com>
Reviewed-by: Zsolt Parragi <zsolt(dot)parragi(at)percona(dot)com>
Reviewed-by: Álvaro Herrera <alvherre(at)kurilemu(dot)de> (in an earlier version)
Discussion: https://www.postgresql.org/message-id/flat/20200612232810.f46nbqkdhbutzqdg%40alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/16fca482548382c8d6255319b1c6e5de4afff7f1

Modified Files
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doc/src/sgml/config.sgml | 6 ++
doc/src/sgml/ref/pgtesttiming.sgml | 153 ++++++++++++++++++++++++++------
src/bin/pg_test_timing/pg_test_timing.c | 105 ++++++++++++++++++++--
src/include/portability/instr_time.h | 7 ++
4 files changed, 233 insertions(+), 38 deletions(-)