Simulator-specific instructions are very useful, we can:

  - Place breakpoints that enable the simulator's interactive debugger, allowing
    us to see registers, the stack and print JS objects.

  - Enable and disable simulator tracing dynamically.

  - Call printf() directly, as the simulator cannot easily support its calling
    convention.

However these tools are not available when generating builtins. The reason is
that when cross-compiling, builtins are generated for real hardware but may
still run inside the simulator on the host if we have a custom snapshot. Using
the `v8_embed_script` GN option will do that for example but embedders may also
do this with the V8 API.

mksnapshot cannot tell the difference between generating code for a simulator
build and a cross-build. If we change this, we can allow us to use
simulator-specific features in builtins in simulator builds.

So in this patch we:

  - Introduce a --target_is_simulator mksnapshot flag to drive the
    enable_simulator_code Assembler option.

  - Make sure the assembler respect the option instead of the USE_SIMULATOR
    macro.


Change-Id: I7a7249f514427c1a2518a1af3679679596a72c7e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1991497Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Pierre Langlois <pierre.langlois@arm.com>
Cr-Commit-Position: refs/heads/master@{#65734}