linux-user/signal: Decode waitid si_code

linux-user/signal: Decode waitid si_code

When mapping the host waitid status to the target status we previously
just used decoding information in the status value. This doesn't follow
what the waitid documentation describes, which instead suggests using
the si_code value for the decoding. This results in the incorrect values
seen when calling waitid. This is especially apparent on RV32 where all
wait calls use waitid (see the bug case).

This patch just passes the waitid status directly back to the guest.

Buglink: https://bugs.launchpad.net/qemu/+bug/1906193
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Tested-by: Andreas K. Hüttel <dilfridge@gentoo.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <1fb2d56aa23a81f4473e638abe9e2d78c09a3d5b.1611080607.git.alistair.francis@wdc.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>