gdbstub: Support disablement in a multi-threaded process

gdbstub: Support disablement in a multi-threaded process

The upcoming follow-fork-mode child support will require disabling
gdbstub in the parent process, which may have multiple threads (which
are represented as CPUs).

Loop over all CPUs in order to remove breakpoints and disable
single-step. Move the respective code into a separate function.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-Id: <20240219141628.246823-2-iii@linux.ibm.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240305121005.3528075-3-alex.bennee@linaro.org>