RFR: JDK-8294266: Add a way to pre-touch java thread stacks

[Thomas Stuefe]

When doing performance- and memory analysis, `AlwaysPreTouch` option is very handy for reducing noise. It would be good to have a similar option for pre-touching thread stacks. In addition to reducing noise, it can serve as worst-case test for thread costs, as well as a test for NMT regressions.

Patch adds a new diagnostic switch, `AlwaysPreTouchStacks`, as a companion switch to `AlwaysPreTouch`. Touching is super-simple using `alloca()`. Also, regression test.

Examples:

NMT, thread stacks, 10000 Threads, default:


-                    Thread (reserved=10332400KB, committed=331828KB)                                       
                            (thread #10021)                                                                 
                            (stack: reserved=10301560KB, committed=300988KB)                                
                            (malloc=19101KB #60755)                                                         
                            (arena=11739KB #20037)                                                          


NMT, thread stacks, 10000 Threads, +AlwaysPreTouchStacks:


-                    Thread (reserved=10332400KB, committed=10284360KB)                        
                            (thread #10021)                                                    
                            (stack: reserved=10301560KB, committed=10253520KB)                 
                            (malloc=19101KB #60755)                                            
                            (arena=11739KB #20037)

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

Commit messages:
 - Use os::pretouch_memory
 - AlwaysPreTouchStacks

Changes: https://git.openjdk.org/jdk/pull/10403/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10403&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294266
  Stats: 119 lines in 4 files changed: 119 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10403.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10403/head:pull/10403

PR: https://git.openjdk.org/jdk/pull/10403