infra: gen-v8-gn.py should work with ABC=<int> defines

Such defines were introduced in a8ea1f12e3 for
- V8_ARRAY_BUFFER_INTERNAL_FIELD_COUNT
- V8_ARRAY_BUFFER_VIEW_INTERNAL_FIELD_COUNT
- V8_PROMISE_INTERNAL_FIELD_COUNT

Since that commit the v8-gn.h includes lines like below that do not
compile well.
```
ifndef V8_ARRAY_BUFFER_INTERNAL_FIELD_COUNT=0
define V8_ARRAY_BUFFER_INTERNAL_FIELD_COUNT=0 1
else
if V8_ARRAY_BUFFER_INTERNAL_FIELD_COUNT=0 != 1
error "V8_ARRAY_BUFFER_INTERNAL_FIELD_COUNT=0 defined but not set to 1"
endif
endif  // V8_ARRAY_BUFFER_INTERNAL_FIELD_COUNT=0
```

Bug: chromium:42204638
Change-Id: I750f6c633b9aae61190c8d431f21c9bbea9297dd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/6338522
Reviewed-by: Darius Mercadier <dmercadier@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Auto-Submit: Alexey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/main@{#99278}

tools/gen-v8-gn.py

@@ -19,22 +19,35 @@ def parse_args():
   parser = argparse.ArgumentParser()
   parser.add_argument('-o', '--output', type=str, action='store',
                       help='Location of header file to generate')
-  parser.add_argument('-p', '--positive-define', type=str, action='append',
-                      help='Externally visibile positive definition')
+  parser.add_argument(
+      '-p',
+      '--positive-define',
+      type=str,
+      action='append',
+      help='Externally visibile positive definition in format NAME or NAME=VALUE'
+  )
   parser.add_argument('-n', '--negative-define', type=str, action='append',
                       help='Externally visibile negative definition')
   args = parser.parse_args()
 
-def generate_positive_definition(out, define):
+
+def generate_positive_definition(out, define_str):
+  # Split the define into name and value if it contains '='
+  if '=' in define_str:
+    define, value = define_str.split('=', 1)
+  else:
+    define = define_str
+    value = "1"  # Default value if not specified
+
   out.write('''
 #ifndef {define}
-#define {define} 1
+#define {define} {value}
 #else
-#if {define} != 1
-#error "{define} defined but not set to 1"
+#if {define} != {value}
+#error "{define} defined but not set to {value}"
 #endif
 #endif  // {define}
-'''.format(define=define))
+'''.format(define=define, value=value))
 
 def generate_negative_definition(out, define):
   out.write('''