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.

Author: stlankes

Cargo.lock

@@ -1640,9 +1640,9 @@ checksum = "95505c38b4572b2d910cecb0281560f54b440a19336cbbcb27bf6ce6adc6f5a8"
 
 [[package]]
 name = "hermit-abi"
-version = "0.3.6"
+version = "0.3.9"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "bd5256b483761cd23699d0da46cc6fd2ee3be420bbe6d020ae4a091e70b7e9fd"
+checksum = "d231dfb89cfffdbc30e7fc41579ed6066ad03abda9e567ccafae602b97ec5024"
 dependencies = [
  "compiler_builtins",
  "rustc-std-workspace-alloc",

library/std/Cargo.toml

@@ -48,7 +48,7 @@ dlmalloc = { version = "0.2.4", features = ['rustc-dep-of-std'] }
 fortanix-sgx-abi = { version = "0.5.0", features = ['rustc-dep-of-std'], public = true }
 
 [target.'cfg(target_os = "hermit")'.dependencies]
-hermit-abi = { version = "0.3.2", features = ['rustc-dep-of-std'], public = true }
+hermit-abi = { version = "0.3.9", features = ['rustc-dep-of-std'], public = true }
 
 [target.'cfg(target_os = "wasi")'.dependencies]
 wasi = { version = "0.11.0", features = ['rustc-dep-of-std'], default-features = false }

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> {