ArmPlatformPkg: Fix target initialisation in cmd_load_symbols.py

ArmPlatformPkg: Fix target initialisation in cmd_load_symbols.py

The debugger in Arm Development Studio 2021.2 doesn't work with
"ec = debugger.getExecutionContext(0)" because it's subsequently unable
to access memory. Fix it by switching to
"ec = debugger.getCurrentExecutionContext()".

The documentation for waitForStop() says:

"It is not needed after a call to stop() because stop() is blocking."

So, remove the call to waitForStop.

Signed-off-by: Rebecca Cran <rebecca@bsdio.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>