RFR: 8246112: Remove build-time and run-time checks for clock_gettime and CLOCK_MONOTONIC

[David Holmes]

We are now confident that we have build-time and runtime support for clock_gettime and CLOCK_MONOTONIC on all our OpenJDK supported POSIX platforms - see bug report for some more details on different OS. Consequently we can simplify a lot of the code in this area and move common code to os_posix.

As of glibc 2.17 the necessary functions are in glibc rather than librt, but we (Oracle at least) aren't yet in position to set our minimum Linux version to support that. We still have supported platforms at glibc 2.12. So to address that we link librt at build time on Linux. This seems to work find for older and more modern Linuxes and also works for the Apline Linux with Musl variant.

The changes are in layered commits:

Step 1: Remove build time checks. SUPPORTS_CLOCK_MONOTONIC is assumed true and removed.
Step 2: make supports_monotonic_clock always true and so remove checking in OS code
Step 3: Replace gettimeofday by clock_gettime(CLOCK_REALTIME)
Step 4: Move shared time functions to os_posix.cpp
Step 5: Alway link librt on Linux so we don't rely on glibc > 2.17

Testing: tiers 1-3 for functional testing
             built and checked (-Xlog:os) on Linux with glibc 2.12 and 2.17,  macOS 10.13.6 and 10.15.7

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Commit messages:
 - Merge branch 'master' into 8246112-mono
 - Alway link librt on Linux so we don't rely on glibc > 2.12
 - Step 4: Move shared time functions to os_posix.cpp
 - Step 3: Replace gettimeofday by clock_gettime(CLOCK_REALTIME)
 - Step 2: make supports_monotonic_clock always true and so remove checking it (in OS code)
 - 8246112: Remove build-time and run-time checks for clock_gettime and CLOCK_MONOTONIC

Changes: https://git.openjdk.java.net/jdk/pull/2090/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2090&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8246112
  Stats: 380 lines in 11 files changed: 54 ins; 299 del; 27 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2090.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2090/head:pull/2090

PR: https://git.openjdk.java.net/jdk/pull/2090