udelay: Use clock_gettime() if available and precise

Instead of calibrating our busy loop against a coarse clock, check if
a precise clock is available and loop against that. The former is unre-
liable by definition on any modern system that may dynamically reclock
the processor.

v2: Apparently _POSIX_MONOTONIC_CLOCK being defined only means that
    the library knows about CLOCK_MONOTONIC. So check for its support
    at runtime and fall back to CLOCK_REALTIME if it's missing.

TEST=Manually added a 10s loop and compared to real time. Run
     on Linux RPi3, Linux x86 and my original use case Linux in
     VirtualBox (Linux host).

Change-Id: I85ad359823875237ada9cd027af3017d62e9a235
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/19391
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: David Hendricks <david.hendricks@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

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