RFR: 8372584: [Linux]: Replace reading proc to get thread user CPU time with clock_gettime

[Jonas Norlinder]

Since kernel v2.6.12 the Linux ABI have had support for encoding the clock types in the last three bits. Setting bit to 001 (CPUCLOCK_VIRT) will result in the kernel returning only user time. POSIX compliant implementations of pthread_getcpuclockid for the Linux kernel defaults to construct a clockid that with 010 (CPUCLOCK_SCHED) set, which return system+user time, which is what the POSIX standard mandates, see POSIX.1-2024/IEEE Std 1003.1-2024 §3.90. This patch joins the family of glibc, musl etc.  that utilities this bit pattern.

This PR also results in improved performance and thus a reduced observer effect, especially for the 100th percentile (max).

Before patch:

Benchmark                  Mode      Cnt  Score    Error  Units
CPUTime.execute          sample  7506555  0.008 ±  0.001  ms/op
CPUTime.execute:p0.00    sample           0.008           ms/op
CPUTime.execute:p0.50    sample           0.008           ms/op
CPUTime.execute:p0.90    sample           0.008           ms/op
CPUTime.execute:p0.95    sample           0.008           ms/op
CPUTime.execute:p0.99    sample           0.012           ms/op
CPUTime.execute:p0.999   sample           0.015           ms/op
CPUTime.execute:p0.9999  sample           0.021           ms/op
CPUTime.execute:p1.00    sample           1.030           ms/op


After patch:

Benchmark                  Mode      Cnt   Score    Error  Units
CPUTime.execute          sample  8984189  ≈ 10⁻³           ms/op
CPUTime.execute:p0.00    sample           ≈ 10⁻³           ms/op
CPUTime.execute:p0.50    sample           ≈ 10⁻³           ms/op
CPUTime.execute:p0.90    sample           ≈ 10⁻³           ms/op
CPUTime.execute:p0.95    sample           ≈ 10⁻³           ms/op
CPUTime.execute:p0.99    sample            0.001           ms/op
CPUTime.execute:p0.999   sample            0.001           ms/op
CPUTime.execute:p0.9999  sample            0.006           ms/op
CPUTime.execute:p1.00    sample            0.054           ms/op


Testing: `java/lang/management/ThreadMXBean/ThreadUserTime.java` and the added microbenchmark.

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Commit messages:
 - Replace proc with fast Linux specific solution

Changes: https://git.openjdk.org/jdk/pull/28556/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28556&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372584
  Stats: 142 lines in 3 files changed: 88 ins; 42 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/28556.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28556/head:pull/28556

PR: https://git.openjdk.org/jdk/pull/28556