linux-user: Implement PR_{GET,SET}_CHILD_SUBREAPER

The "set" prctl passes through integral values.
The "get" prctl returns the value into a pointer.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1929
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
1 file changed