RFR: 8372625: [Linux] Remove unnecessary logic for supports_fast_thread_cpu_time [v2]

[Jonas Norlinder]

> For Linux we test if we can use `CLOCK_THREAD_CPUTIME_ID` but not the clocks supported by `pthread_getcpuclockid()`.
> 
> The following comment has been around for at least 18 years according to the git commit history (points to the initial git commit from Dec 1, 2007):
> 
> 
> // Switch to using fast clocks for thread cpu time if
> // the sys_clock_getres() returns 0 error code.
> // Note, that some kernels may support the current thread
> // clock (CLOCK_THREAD_CPUTIME_ID) but not the clocks
> // returned by the pthread_getcpuclockid().
> // If the fast Posix clocks are supported then the sys_clock_getres()
> // must return at least tp.tv_sec == 0 which means a resolution
> // better than 1 sec. This is extra check for reliability.
> 
> 
> There was a time when the Linux kernel did not provide an implementation for `CLOCK_THREAD_CPUTIME_ID` (that arrived with v2.6.12). During this period, glibc emulated support for `clock_gettime(CLOCK_THREAD_CPUTIME_ID)` in userspace for the current thread only. `clock_gettime(CLOCK_THREAD_CPUTIME_ID)` will call `pthread_getcpuclockid` in https://elixir.bootlin.com/glibc/glibc-2.2.5/source/linuxthreads/sysdeps/pthread/getcpuclockid.c. The checks and comment exists because `CLOCK_THREAD_CPUTIME_ID` worked (via emulation) but `pthread_getcpuclockid()` failed. 
> 
> During that period of the early 2000s this check was indeed needed. But now sufficient time has passed and we don't support OpenJDK for that old kernel versions/glibc. Therefore, this check should be removed and some code can be cleaned up.

Jonas Norlinder has updated the pull request incrementally with one additional commit since the last revision:

  Remove dlsym and fix to correct return statement

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/28516/files
  - new: https://git.openjdk.org/jdk/pull/28516/files/36934858..558b492d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28516&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28516&range=00-01

  Stats: 9 lines in 2 files changed: 0 ins; 7 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/28516.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28516/head:pull/28516

PR: https://git.openjdk.org/jdk/pull/28516