[musl] Remove linux syscalls and stub some io functions

- Posix message queues are essentially unused, and there is nothing
interesting to keep in the current implementation should we want it.

- wait3 and wait4 are nonstandard.

- chroot is nonstandard.

- We want to just directly call open() and close(). Note that there is
a little bit of careful handling of return values around close(), as
the original syscall wrappers managed the syscall and the conversion
from return-a-negative-error to return-negative-one-and-set-errno in
stages.

Change-Id: If574b115cc3a493d192530866d605bc12a3f5cb8

[magenta] Initial commit