add platform-specific function to get the error number for HermitOS

Extending `std` to get the last error number for HermitOS.

HermitOS is a tier 3 platform and this PR changes only files,
wich are related to the tier 3 platform.

library/std/src/sys/pal/hermit/os.rs

@@ -14,15 +14,15 @@ use crate::sys::unsupported;
 use crate::vec;
 
 pub fn errno() -> i32 {
-    0
+    unsafe { abi::get_errno() }
 }
 
-pub fn error_string(_errno: i32) -> String {
-    "operation successful".to_string()
+pub fn error_string(errno: i32) -> String {
+    abi::error_string(errno).to_string()
 }
 
 pub fn getcwd() -> io::Result<PathBuf> {
-    unsupported()
+    Ok(PathBuf::from("/"))
 }
 
 pub fn chdir(_: &path::Path) -> io::Result<()> {
@@ -188,7 +188,7 @@ pub fn unsetenv(k: &OsStr) -> io::Result<()> {
 }
 
 pub fn temp_dir() -> PathBuf {
-    panic!("no filesystem on hermit")
+    PathBuf::from("/tmp")
 }
 
 pub fn home_dir() -> Option<PathBuf> {