gdbstub: Adjust gdb_syscall_complete_cb declaration

Change 'ret' to uint64_t.  This resolves a FIXME in the
m68k and nios2 semihosting that we've lost data.
Change 'err' to int.  There is nothing target-specific
about the width of the errno value.

Reviewed-by: Luc Michel <lmichel@kalray.eu>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>