[musl] Remove empty and nonstandard syscall.h files

These headers make some sense on unix-y systems where the kernel
interface is intended to be a stable one. Magenta's syscall interface
is explictly not a stable API.

Change-Id: I94564fbfa662d072c916d53c5518c91ebd85c532

Revert "[musl] Remove empty and nonstandard syscall.h files"

This reverts commit a33a52bfa393fc2fcd3ca527cbda80ebdcb9640e.

Reason for revert: Some Fuchsia code I was not aware of was using this. I'll fix that and resubmit this.

Original change's description:
> [musl] Remove empty and nonstandard syscall.h files
>
> These headers make some sense on unix-y systems where the kernel
> interface is intended to be a stable one. Magenta's syscall interface
> is explictly not a stable API.
>
> Change-Id: I49f52be779261e981c2274f0acae95ba10553252
>

TBR=kulakowski@google.com,swetland@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Change-Id: If594a9a1d7d2cc30ff9de022fa7912a0d23b92ed

[musl] Remove empty and nonstandard syscall.h files

These headers make some sense on unix-y systems where the kernel
interface is intended to be a stable one. Magenta's syscall interface
is explictly not a stable API.

Change-Id: I49f52be779261e981c2274f0acae95ba10553252

[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] clang format musl

Change-Id: I3bc6ab7655410514691cd07f08454f3afd41d9b3

[magenta] Initial commit