[musl] Remove legacy linux syscall mechanism entirely

Change-Id: Ia013bac87e87a7a4528b496ac26da649ad130da0

[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

[musl] Remove linux-y LFS64 abstraction and xstat extension

Change-Id: Id54c438c2b36d7fce3adb9bd7d32045bbc0765fb

[musl] Move unlink, unlinkat, and rmdir to __libc_io_*

Change-Id: I1ff2c4e279dde180b46ef988548a3e074bc23979

[musl] clang format musl

Change-Id: I3bc6ab7655410514691cd07f08454f3afd41d9b3

[magenta] Initial commit