semihosting: Return void from do_common_semihosting

semihosting: Return void from do_common_semihosting

Perform the cleanup in the FIXME comment in common_semi_gdb_syscall.
Do not modify guest registers until the syscall is complete,
which in the gdbstub case is asynchronous.

In the synchronous non-gdbstub case, use common_semi_set_ret
to set the result.  Merge set_swi_errno into common_semi_cb.
Rely on the latter for combined return value / errno setting.

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