RFR(S) 8242504: Enhance the system clock to nanosecond precision

[David Holmes]

bug: https://bugs.openjdk.java.net/browse/JDK-8242504
webrev: http://cr.openjdk.java.net/~dholmes/8242504/webrev/

This work was contributed by Mark Kralj-Taylor:

https://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2020-April/038975.html

On the hotspot side we change the Linux implementations of 
javaTimeMillis() and javaTimeSystemUTC() so that they use 
clock_gettime(CLOCK_REALTIME) instead of gettimeofday(). In keeping with 
our conditional use of this API I added a new guard

os::Posix::supports_clock_gettime()

and refined the existing supports_monotonic_clock() so that we can still 
use CLOCK_REALTIME if CLOCK_MONOTONIC does not exist. In the future 
(hopefully very near future) we will simply assume these APIs always exist.

On the core-libs side the existing test:

test/jdk/java/time/test/java/time/TestClock_System.java

is adjusted to track the precision in more detail.

Finally Mark has added instantNowAsEpochNanos() to the benchmark 
previously known as

test/micro/org/openjdk/bench/java/lang/Systems.java

which I've renamed to SystemTime.java as Mark suggested. I agree having 
these three functions measured together makes sense.

Testing:
   - test/jdk/java/time/test/java/time/TestClock_System.java
   - test/micro/org/openjdk/bench/java/lang/SystemTime.java
   - Tiers 1-3

Thanks,
David