Closes #1055.

Non-existent constants like SYS_syscalls have been removed

The second argument of `gettimeofday` was a `*mut c_void` on all targets,
but that type is incorrect in the following targets, where it should be
a `*mut timezone` instead:

On these other targets it appears that the signature of gettimeofday was incorrect (it takes a time-zone pointer instead of a void pointer):

linux+gnu: http://man7.org/linux/man-pages/man2/gettimeofday.2.html
freebsd: https://www.freebsd.org/cgi/man.cgi?query=gettimeofday&apropos=0&sektion=2&manpath=FreeBSD+11.2-stable&arch=default&format=html
openbsd: https://man.openbsd.org/gettimeofday.2
android: https://github.com/ricardoquesada/android-ndk/blob/master/usr/include/sys/time.h
dragonfly: https://www.dragonflybsd.org/cgi/web-man?command=gettimeofday&section=2

This commit corrects the type on these targets, which is a breaking change. Due
to how this API is commonly used (e.g. passing `ptr::null_mut` to the second
argument), breakage should be minimal. Users wanting to support both versions
can just write `ptr as *mut _` instead.

Closes #1338.

Because we cannot yet bring a more recent musl to the mips and mipsel
architectures, we disable support for these constant until a cascading
update with rust-lang/rust has been done.

They are defined since Linux 3.1 but not in musl yet.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

 - Rename `wasm32-unknown-wasi` to `wasm32-wasi`.
 - `__wasilibc_rmfileat` was renamed to `__wasilibc_unlinkat`
 - Add bindings for a few more functions and typedefs.

See https://github.com/rust-lang/rust/pull/60775 .