RFR: 8344363: FullGCForwarding::initialize_flags is called after ObjLayout::initialize

[Roman Kennke]

>From the bug description:
ObjLayout::initialize() is called in Arguments::parse(const JavaVMInitArgs*) which sets ObjLayout::_klass_mode. FullGCForwarding::initialize_flags(size_t) is called in init_globals() which seems to be later in the Threads::create_vm(JavaVMInitArgs*, bool*) routine. The latter, however, can unset the UseCompactObjectHeaders flag, which leads to a potential mismatch with ObjLayout::_klass_mode, firing the asserts in ObjLayout::klass_mode().

There is also a related (and somewhat minor) problem: In Arguments::parse() we enable a bunch of stuff when UseCompactObjectHeaders is on (LW locking, but that's the default anyway, and object-monitor-tables, which are otherwise off), but then may disable UseCompactObjectHeaders, e.g. in the GC or when -UseCompressedClassPointers is requested. This then leaves the odd situation that we run without compact headers, but have object monitor tables turned on.

The fix to both issues is:
- First disable UCOH
- and do that in apply_ergo() (e.g. GCs should do it in GCArguments::initialize())
- then enable any flags required by compact headers
- Initialize ObjLayout after all flags are done (it's in the correct place, already)

Testing:
 - [x] tier1 -UCOH (default)
 - [ ] tier1 +UCOH
 - [x] Manual testing several flag combinations

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Commit messages:
 - 8344363: FullGCForwarding::initialize_flags is called after ObjLayout::initialize

Changes: https://git.openjdk.org/jdk/pull/22255/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22255&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344363
  Stats: 35 lines in 9 files changed: 14 ins; 16 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/22255.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22255/head:pull/22255

PR: https://git.openjdk.org/jdk/pull/22255