fix: add RiscvInterrupt* cconv to smir

These new interrupt calling conventions are not themselves stabilized,
but there are other unstable calling conventions present in the SMIR
mapping (e.g. AVR interrupts) and the mapping appears to be "complete"
so far, with no obvious way to represent unstable conventions separately
from the stable ones.

compiler/rustc_smir/src/rustc_smir/mod.rs

@@ -877,6 +877,8 @@ impl<'tcx> Stable<'tcx> for ty::FnSig<'tcx> {
                 abi::Abi::PlatformIntrinsic => Abi::PlatformIntrinsic,
                 abi::Abi::Unadjusted => Abi::Unadjusted,
                 abi::Abi::RustCold => Abi::RustCold,
+                abi::Abi::RiscvInterruptM => Abi::RiscvInterruptM,
+                abi::Abi::RiscvInterruptS => Abi::RiscvInterruptS,
             },
         }
     }

compiler/rustc_smir/src/stable_mir/ty.rs

@@ -178,6 +178,8 @@ pub enum Abi {
     PlatformIntrinsic,
     Unadjusted,
     RustCold,
+    RiscvInterruptM,
+    RiscvInterruptS,
 }
 
 #[derive(Clone, Debug)]