[Linux]: faster getCurrentThreadUserTime()

[jerrinot]

This PR further optimizes os::current_thread_cpu_time (used by ThreadMXBean.getCurrentThreadUserTime()) on Linux by using the kernel's fast-path for the calling thread. I see it as a continuation of #28556.

The current code in master has 2 phases:

1. Create a timer bound to the current Thread ID (TID) + massage the timer to switch from CPUCLOCK_SCHED timer to CPUCLOCK_VIRT
2. Use the timer in clock_gettime()

Within the kernel, a routine exists to extract the TID from the timer, followed by a radix lookup to find booking structures for the specified TID.

However, the kernel also has a fast-path: When TID is 0, then the kernel avoids the radix lookup and instead treats the timer as bound to 'the current thread' (or a process, depending on lower bits of the clock). The current thread/task has bookkeeping structures more readily available and the radix lookup is avoided. The result is around 13% faster getCurrentThreadUserTime().

Potential concerns

1. ABI stability. I believe Linux policy on ABI stability is simple: "WE DO NOT BREAK USERSPACE!" Changing the behaviour when TID is 0 would break user-space.
2. I am not fluent in C++ and I am not entirely sure if the expression to build the timer is UB-free. Perhaps there is a better way which does not rely on a specific size of clockid_t. Feedback here is very much appreciated.

The benchmark from #28556 (switched to nanos + more iterations + fork count):

before:
Benchmark                                             Mode      Cnt       Score   Error  Units
ThreadMXBeanBench.getCurrentThreadUserTime          sample  4347067      81.746 ± 0.510  ns/op
ThreadMXBeanBench.getCurrentThreadUserTime:p0.00    sample               69.000          ns/op
ThreadMXBeanBench.getCurrentThreadUserTime:p0.50    sample               80.000          ns/op
ThreadMXBeanBench.getCurrentThreadUserTime:p0.90    sample               90.000          ns/op
ThreadMXBeanBench.getCurrentThreadUserTime:p0.95    sample               90.000          ns/op
ThreadMXBeanBench.getCurrentThreadUserTime:p0.99    sample               90.000          ns/op
ThreadMXBeanBench.getCurrentThreadUserTime:p0.999   sample              230.000          ns/op
ThreadMXBeanBench.getCurrentThreadUserTime:p0.9999  sample             1980.000          ns/op
ThreadMXBeanBench.getCurrentThreadUserTime:p1.00    sample           653312.000          ns/op

after:
Benchmark                                             Mode      Cnt       Score   Error  Units
ThreadMXBeanBench.getCurrentThreadUserTime          sample  5081223      70.813 ± 0.325  ns/op
ThreadMXBeanBench.getCurrentThreadUserTime:p0.00    sample               59.000          ns/op
ThreadMXBeanBench.getCurrentThreadUserTime:p0.50    sample               70.000          ns/op
ThreadMXBeanBench.getCurrentThreadUserTime:p0.90    sample               70.000          ns/op
ThreadMXBeanBench.getCurrentThreadUserTime:p0.95    sample               70.000          ns/op
ThreadMXBeanBench.getCurrentThreadUserTime:p0.99    sample               80.000          ns/op
ThreadMXBeanBench.getCurrentThreadUserTime:p0.999   sample              170.000          ns/op
ThreadMXBeanBench.getCurrentThreadUserTime:p0.9999  sample             1830.000          ns/op
ThreadMXBeanBench.getCurrentThreadUserTime:p1.00    sample           425472.000          ns/op

@JonasNorlinder: this is what I raised on the mailing-list.

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