RFR: 8345265: Minor improvements for LTO across all compilers [v3]

[Julian Waters]

> This is a general cleanup and improvement of LTO, as well as a quick fix to remove a workaround in the Makefiles that disabled LTO for g1ParScanThreadState.cpp due to the old poisoning mechanism causing trouble. The -Wno-attribute-warning change here can be removed once Kim's new poisoning solution is integrated.
> 
> - -fno-omit-frame-pointer is added to gcc to stop the linker from emitting code without the frame pointer
> - -flto is set to $(JOBS) instead of auto to better match what the user requested
> - -Gy is passed to the Microsoft compiler. This does not fully fix LTO under Microsoft, but prevents warnings about -LTCG:INCREMENTAL at least

Julian Waters has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 21 commits:

 - Revert recent changes to ClientLibraries.gmk
 - Revert recent changes to CompileJvm.gmk
 - Revert recent changes to Flags.gmk
 - Revert recent changes to NativeCompilation.gmk
 - Revert recent changes to spec.gmk.template
 - Revert recent changes to flags-ldflags.m4
 - Revert recent changes to flags-cflags.m4
 - Remove no longer needed warning disable in JvmFeatures.gmk
 - Merge branch 'master' into patch-16
 - Merge branch 'openjdk:master' into patch-16
 - ... and 11 more: https://git.openjdk.org/jdk/compare/34d6cc0d...9599d09e

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Changes: https://git.openjdk.org/jdk/pull/22464/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22464&range=02
  Stats: 53 lines in 9 files changed: 11 ins; 38 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/22464.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22464/head:pull/22464

PR: https://git.openjdk.org/jdk/pull/22464