Generates callbacks for already existent code by using the
kJitCodeEventEnumExisting option instead of kJitCodeEventDefault.

When working on V8 system instrumentation on Windows, it was found in
https://bugs.chromium.org/p/v8/issues/detail?id=11043#c41 that several
parts of the JS stack was missing.

One missing part is all calls to Builtin methods
(example: Builtin:JSEntryTrampoline) and this change ensures that
events with name "Builtin" are also included in the generated JIT code
events.

Bug: v8:11043
Change-Id: Iaa99e18d799266a7224f848130c4eaf36e8c77fb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3550590Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Henrik Andreasson <henrika@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79640}