[musl] Remove legacy linux syscall mechanism entirely

Change-Id: Ia013bac87e87a7a4528b496ac26da649ad130da0

[musl] Make stdio FILE* close actually close again

Without this change, it was easy to observe fds leaking (for instance by
listing files from mxsh over and over), and with this change the fd is
reclaimed so the number is always the same.

MG-256 #comment fixes

Change-Id: I187f42e35d6b5a3a89c6ba1a9580c73a78b52280

[musl] Remove aio support from musl

This isn't particularly commonly used, and this is the sort of thing
we'd want people to use the better Fuchsia native APIs for anyway.

Change-Id: I980f5fa50af4deb6e14843884056565ddbd10c23

[magenta] Initial commit