[v24.x] build,src: gate jspi flag on V8_ENABLE_WEBASSEMBLY

[pkubaj]

Since v24.20.0 (backport of #59941) ProcessGlobalArgsInternal() pushes
--experimental-wasm-jspi unconditionally. With ./configure --v8-lite-mode,
v8_enable_webassembly is 0, V8 is built without V8_ENABLE_WEBASSEMBLY, and
the whole experimental_wasm_* flag block in flag-definitions.h does not
exist. V8::SetFlagsFromCommandLine() is called with remove_flags=true, which
silently leaves unknown flags in argv, so node reports them as bad options and
exits with kInvalidCommandLineArgument. The first binary to run that code is
node_mksnapshot, so every lite-mode build fails:

bad option: --experimental-wasm-jspi
node_mksnapshot failed with exit code 9

This forwards v8_enable_webassembly to all targets as
V8_ENABLE_WEBASSEMBLY, the same way common.gypi already forwards the other
V8 feature defines, adds the % default the file needs for node-gyp, and only
pushes the JSPI flag when WebAssembly is compiled in. V8's public headers do not
reference V8_ENABLE_WEBASSEMBLY, so defining it for node's own sources does
not affect the V8 API.

--v8-lite-mode is not covered by CI, which is why this was not caught.

Verification

• FreeBSD/amd64, ./configure --v8-lite-mode (the www/node24 port with the
  JIT option off): node_mksnapshot now succeeds and the build completes;
  node -p 'typeof WebAssembly' prints undefined as expected for lite mode.
• Default configure: unchanged; node -p 'typeof WebAssembly.Suspending'
  still prints function, so JSPI remains enabled.

AI disclosure

The root-cause analysis and the patch were drafted with Claude (Anthropic's
coding agent). I verified the analysis against deps/v8/src/flags/flag-definitions.h,
deps/v8/src/flags/flags.cc, configure.py and tools/v8_gypfiles/features.gypi,
and build-tested both configurations listed above myself.

Refs: #59941

[nodejs-github-bot]

Review requested:

• @nodejs/gyp
• @nodejs/startup

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.32%. Comparing base (9e39360) to head (b44cdd0).
⚠️ Report is 583 commits behind head on v24.x-staging.

Additional details and impacted files

@@                Coverage Diff                @@
##           v24.x-staging   #66082      +/-   ##
=================================================
+ Coverage          90.31%   90.32%   +0.01%     
=================================================
  Files                711      714       +3     
  Lines             228425   231856    +3431     
  Branches           43160    43953     +793     
=================================================
+ Hits              206293   209427    +3134     
- Misses             14090    14151      +61     
- Partials            8042     8278     +236     

Files with missing lines	Coverage Δ
src/node.cc	73.90% <ø> (-0.59%)	⬇️

... and 170 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/77963/

[aduh95]

Why does this target v24.x-staging and not main?

[pkubaj]

The bug only exists on v24.x. On main the push from #59941 (4396cf2) was reverted five days later in #60014 (23b8340, "revert enable JSPI as already enabled"): main's V8 ships JSPI unconditionally and no longer has an experimental_wasm_jspi flag, so there is nothing to gate.

[aduh95]

Landed in 249e9a1