Merge bitcoin/bitcoin#31191: build: Make G_FUZZING constexpr, require -DBUILD_FOR_FUZZING=ON to fuzz

fafbf8acf4 Make G_FUZZING constexpr, require -DBUILD_FOR_FUZZING=ON to execute a fuzz target (MarcoFalke)
fae3cf0ffa ci: Temporarily disable macOS/Windows fuzz step (MarcoFalke)

Pull request description:

  `g_fuzzing` is used inside `Assume` at runtime, causing significant overhead in hot paths. See https://github.com/bitcoin/bitcoin/issues/31178

  One could simply remove the `g_fuzzing` check from the `Assume`, but this would make fuzzing a bit less useful. Also, it would be unclear if `g_fuzzing` adds a runtime overhead in other code paths today or in the future.

  Fix all issues by making `G_FUZZING` equal to the build option `BUILD_FOR_FUZZING`, and for consistency in fuzzing, require it to be set when executing any fuzz target.

  Fixes https://github.com/bitcoin/bitcoin/issues/31178

  Temporarily this drops fuzzing from two CI tasks, but they can be re-added in a follow-up with something like https://github.com/bitcoin/bitcoin/pull/31073

ACKs for top commit:
  marcofleon:
    Tested ACK fafbf8acf4
  davidgumberg:
    I still ACK fafbf8acf4 for fixing the regression measured in #31178.
  ryanofsky:
    Code review ACK fafbf8acf4 but approach -0, because this approach means libraries built for fuzz testing do not function correctly if used in a release, and libraries built for releases are mostly useless for fuzz testing. So I would like to at least consider other solutions to this problem even if we go with this one.
  dergoegge:
    utACK fafbf8acf4

Tree-SHA512: 124fc2e8b35e0c4df414436556a7a0a36cd1bec4b3000b40dcf2ab8c85f32e0610bf7f70d2fd79223d62f3c3665b6c09da21241654c7b9859461b8ca340d5421

.github/workflows/ci.yml

@@ -212,20 +212,6 @@ jobs:
         shell: cmd
         run: py -3 test\functional\test_runner.py --jobs %NUMBER_OF_PROCESSORS% --ci --quiet --tmpdirprefix=%RUNNER_TEMP% --combinedlogslen=99999999 --timeout-factor=%TEST_RUNNER_TIMEOUT_FACTOR% %TEST_RUNNER_EXTRA%
 
-      - name: Clone fuzz corpus
-        run: |
-          git clone --depth=1 https://github.com/bitcoin-core/qa-assets "$env:RUNNER_TEMP\qa-assets"
-          Set-Location "$env:RUNNER_TEMP\qa-assets"
-          Write-Host "Using qa-assets repo from commit ..."
-          git log -1
-
-      - name: Run fuzz binaries
-        working-directory: build
-        env:
-          BITCOINFUZZ: '${{ github.workspace }}\build\src\test\fuzz\Release\fuzz.exe'
-        shell: cmd
-        run: py -3 test\fuzz\test_runner.py --par %NUMBER_OF_PROCESSORS% --loglevel DEBUG %RUNNER_TEMP%\qa-assets\fuzz_corpora
-
   asan-lsan-ubsan-integer-no-depends-usdt:
     name: 'ASan + LSan + UBSan + integer, no depends, USDT'
     runs-on: ubuntu-24.04 # has to match container in ci/test/00_setup_env_native_asan.sh for tracing tools