gdbstub: Expose TARGET_SIGTRAP in a target-agnostic way

gdbstub: Expose TARGET_SIGTRAP in a target-agnostic way

The upcoming syscall catchpoint support needs to send SIGTRAP stop
packets to GDB. Being able to compile this support only once for all
targets is a good thing, and it requires hiding TARGET_SIGTRAP behind
a function call.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-Id: <20240202152506.279476-2-iii@linux.ibm.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240207163812.3231697-11-alex.bennee@linaro.org>