gdbstub: split out softmmu/user specifics for syscall handling

gdbstub: split out softmmu/user specifics for syscall handling

Most of the syscall code is config agnostic aside from the size of
target_ulong. In preparation for the next patch move the final bits
of specialisation into the appropriate user and softmmu helpers.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230302190846.2593720-26-alex.bennee@linaro.org>
Message-Id: <20230303025805.625589-26-richard.henderson@linaro.org>