CmdLine: Drop the legacy cpu core flags, use --cpu=

-i, -j, -r, -J, --ir and --jit-ir are gone. -r was also --root's short form,
and --root won, so -r took the next argument as the root path and nothing
booted (test.py -r failed every test).

-i, -j and -J now fail with an error pointing at --cpu= instead of quietly
running on the default core. Docs and test.py updated.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Core/Debugger/WebSocket/BreakpointSubscriber.cpp

@@ -750,7 +750,7 @@ struct WebSocketRegBreakpointParams {
 // Add a new register write breakpoint (cpu.regBreakpoint.add)
 //
 // Interpreter-only for now - see RegBreakpoint in Core/Debugger/Breakpoints.h. Has no effect
-// while running under a JIT backend (force the interpreter core, e.g. -i on the command line).
+// while running under a JIT backend (force the interpreter core, e.g. --cpu=interpreter on the command line).
 //
 // Parameters:
 //  - register: unsigned integer 0-31 GPR index to break on write to. Ignored if name given.