RFR: 8371381: [Shenandoah] Setting ergo flags should use FLAG_SET_ERGO [v3]

[Rui Li]

> Setting ergo flags using `FLAG_SET_ERGO`, instead of `FLAG_SET_DEFAULT`, so we can have the right origin info.
> 
> 
> For `ShenandoahEvacReserve` change: when we use shenandoah passive mode and degen is also turned off (`-XX:ShenandoahGCMode=passive -XX:-ShenandoahDegeneratedGC`), we set the ShenandoahEvacReserve to 0:  https://github.com/openjdk/jdk/blob/c6a8027b94bbcbde5f7dcabd0bff48b93bbb5a7f/src/hotspot/share/gc/shenandoah/mode/shenandoahPassiveMode.cpp#L40-L42
> 
> `FLAG_SET_DEFAULT` doesn't check the range but `FLAG_SET_ERGO` does so some of the jtreg would fail because of this change. Had to move this ergo setting to regular `FLAG_SET_DEFAULT`.
> 
> Testing: jtreg gc. GHA pending.

Rui Li has updated the pull request incrementally with one additional commit since the last revision:

  Move ShenandoahEvacReserve back to (1,100)

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/28242/files
  - new: https://git.openjdk.org/jdk/pull/28242/files/65fc9878..6200456b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28242&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28242&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/28242.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28242/head:pull/28242

PR: https://git.openjdk.org/jdk/pull/28242