`GETRUSAGE_BASED_CLOCK_PROCESS_CPUTIME_ID` clock uses `getrusage`
always if available as the name states.  That is if it is implemented
`getrusage` is available, regardless microseconds in its results.

Prior to commit 5806c54447439f2ba22892e4045e78dd80f96f0c, it was "at
least one result with precision beyond milliseconds (with none-zero
microseconds) should exist"; after this commit, "at least one result
should have zero microseconds".  This chance is lower than the
previous condition.

This reverts commit 87253d047ce35e7836b6f97edbb4f819879a3b25.

Revert "Implement `Process.warmup`"

This reverts commit ba6ccd871442f55080bffd53e33678c0726787d2.

[Feature #18885]

For now, the optimizations performed are:

  - Run a major GC
  - Compact the heap
  - Promote all surviving objects to oldgen

Other optimizations may follow.