Extract out the command processing from Simulator::Debug(), and expose
it to gdb as a new 'sim' command. Example usage:

    (gdb) sim p x15
    (gdb) sim stack

The sim command will execute that one command, and will return to gdb.

For a list of all commands, you can call

    (gdb) sim help

Note that sim won't resume simulator execution until gdb continues
execution; for example, `sim next` will set a breakpoint on the next
instruction, and will return to gdb. The user then has to continue
execution in gdb, at which point the simulator will break. The user can
then re-enter gdb with the gdb command. This will look like this:

    (gdb) sim next
    (gdb) continue
    ...
    sim> gdb
    (gdb) ...

Change-Id: I678e71e2642d8427950b5f7ed65890ceae69e18d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2664448
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72479}