Couple of changes to make it easier to compile rustc for wasm

This is a subset of the patches I have on my rust fork to compile rustc
for wasm32-wasip1.

compiler/rustc_fs_util/src/lib.rs

@@ -76,10 +76,14 @@ pub fn link_or_copy<P: AsRef<Path>, Q: AsRef<Path>>(p: P, q: Q) -> io::Result<Li
     }
 }
 
-#[cfg(unix)]
+#[cfg(any(unix, all(target_os = "wasi", target_env = "p1")))]
 pub fn path_to_c_string(p: &Path) -> CString {
     use std::ffi::OsStr;
+    #[cfg(unix)]
     use std::os::unix::ffi::OsStrExt;
+    #[cfg(all(target_os = "wasi", target_env = "p1"))]
+    use std::os::wasi::ffi::OsStrExt;
+
     let p: &OsStr = p.as_ref();
     CString::new(p.as_bytes()).unwrap()
 }

src/bootstrap/src/utils/shared_helpers.rs

@@ -49,6 +49,8 @@ pub fn exe(name: &str, target: &str) -> String {
         format!("{name}.exe")
     } else if target.contains("uefi") {
         format!("{name}.efi")
+    } else if target.contains("wasm") {
+        format!("{name}.wasm")
     } else {
         name.to_string()
     }