account for system abi (#2586)

* account for system abi

Closes #2576

* add test for system abi

* Update changelog

---------

Co-authored-by: Christian Poveda <christian.poveda@ferrous-systems.com>

CHANGELOG.md

@@ -171,6 +171,7 @@
 # Unreleased
 
 ## Added
+- The `system` ABI is now supported as an option for the `--override-abi` flag.
 ## Changed
 - The `Clone` implementation for `_BindgenUnionField` has been changed to pass
   the `incorrect_clone_impl_on_copy_type` Clippy lint.

bindgen-tests/tests/headers/abi-override.h

@@ -1,8 +1,9 @@
-// bindgen-flags: --override-abi=foo=fastcall --override-abi=bar=stdcall --override-abi=boo=efiapi --override-abi=foobar=efiapi
+// bindgen-flags: --override-abi=foo=fastcall --override-abi=bar=stdcall --override-abi=boo=efiapi --override-abi=foobar=efiapi --override-abi qux=system
 
 void foo();
 void bar();
 void baz();
+void qux();
 
 typedef void (*boo)();
 typedef void (*foobar)(boo boo);

bindgen/ir/function.rs

@@ -190,6 +190,8 @@ pub enum Abi {
     Win64,
     /// The "C-unwind" ABI.
     CUnwind,
+    /// The "system" ABI.
+    System,
 }
 
 impl FromStr for Abi {
@@ -206,6 +208,7 @@ impl FromStr for Abi {
             "aapcs" => Ok(Self::Aapcs),
             "win64" => Ok(Self::Win64),
             "C-unwind" => Ok(Self::CUnwind),
+            "system" => Ok(Self::System),
             _ => Err(format!("Invalid or unknown ABI {:?}", s)),
         }
     }
@@ -223,6 +226,7 @@ impl std::fmt::Display for Abi {
             Self::Aapcs => "aapcs",
             Self::Win64 => "win64",
             Self::CUnwind => "C-unwind",
+            Abi::System => "system",
         };
 
         s.fmt(f)