[musl] Add remaining missing calls to iostubs

This also moves remove and creat back to libc, as they can call unlink
and open respectively without needing to know anything about how mxio
works.

With this change, all libc or posix functions defined in mxio's unistd.c
and stubs.c are in also in iostubs.c.

Change-Id: Ibf201a6da2155c6c9ed914a20c9a586eed4dd2ee

[musl][mxio] migrate a bunch of fileio functions to stubs in mxio

Change-Id: I404316b19cd0dddfcc6f37ec005c7c00cd226f31

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

Change-Id: Id54c438c2b36d7fce3adb9bd7d32045bbc0765fb

[magenta] Initial commit