build: add -flax-vector-conversions to V8 build

The flag is on by default in Clang and V8 recently made a change that
makes it necessary with GCC.

Refs: 7fbbf93ea8
Refs: 54067c5fbe/clang/include/clang/Basic/LangOptions.def (L133-L134)
Refs: https://gcc.gnu.org/onlinedocs/gcc/C-Dialect-Options.html#index-flax-vector-conversions
PR-URL: https://github.com/nodejs/node/pull/51257
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>

tools/v8_gypfiles/toolchain.gypi

@@ -138,7 +138,11 @@
         'cflags': [ '-Werror', '-Wno-unknown-pragmas' ],
       },{
         'cflags!': [ '-Wall', '-Wextra' ],
-        'cflags': [ '-Wno-return-type' ],
+        'cflags': [
+          '-Wno-return-type',
+          # On by default in Clang and V8 requires it at least for arm64.
+          '-flax-vector-conversions',
+        ],
       }],
       ['clang or OS!="win"', {
         'cflags': [ '-Wno-invalid-offsetof' ],