[wasi-common]: add armv7 support to wasi-common (#1269)

* Add armv7 support to wasi-common

This commit enables `target_pointer_width = 32` compatibility for
`wasi-common` (and by transitivity, any crate found inside, e.g., `yanix`).
I've also added a simplistic (bare minimum) check to our CI to ensure
that `wasi-common` cross-compiles to `armv7-unknown-gnueabihf` fine.
While here, I've done the same for `wasm32-unknown-emscripten`.

* Clean arch-specific impls + reuse libc consts

* Make SeekLoc::from_raw platform independent

* Collapse CI cc jobs into one

crates/wasi-common/yanix/src/dir.rs

@@ -2,7 +2,8 @@ use crate::{
     file::FileType,
     sys::dir::{iter_impl, EntryImpl},
 };
-use std::io::Result;
+use std::convert::TryInto;
+use std::io::{Error, Result};
 use std::os::unix::io::{AsRawFd, IntoRawFd, RawFd};
 use std::{ffi::CStr, io, ops::Deref, ptr};
 
@@ -98,6 +99,18 @@ impl SeekLoc {
     pub fn to_raw(&self) -> i64 {
         self.0.into()
     }
+
+    pub unsafe fn from_raw(loc: i64) -> Result<Self> {
+        // The cookie (or `loc`) is an opaque value, and applications aren't supposed to do
+        // arithmetic on them or pick their own values or have any awareness of the numeric
+        // range of the values. They're just supposed to pass back in the values that we
+        // give them. And any value we give them will be convertable back to `long`,
+        // because that's the type the OS gives them to us in. So return an `EINVAL`.
+        let loc = loc
+            .try_into()
+            .map_err(|_| Error::from_raw_os_error(libc::EINVAL))?;
+        Ok(Self(loc))
+    }
 }
 
 #[derive(Debug)]