Add TargetIsa::map_dwarf_register; fixes #1471

This exposes the functionality of `fde::map_reg` on the `TargetIsa` trait, avoiding compilation errors on architectures where register mapping is not yet supported. The change is conditially compiled under the `unwind` feature.

cranelift/codegen/src/isa/x86/mod.rs

@@ -22,6 +22,8 @@ use crate::binemit::{FrameUnwindKind, FrameUnwindSink};
 use crate::ir;
 use crate::isa::enc_tables::{self as shared_enc_tables, lookup_enclist, Encodings};
 use crate::isa::Builder as IsaBuilder;
+#[cfg(feature = "unwind")]
+use crate::isa::{fde::RegisterMappingError, RegUnit};
 use crate::isa::{EncInfo, RegClass, RegInfo, TargetIsa};
 use crate::regalloc;
 use crate::result::CodegenResult;
@@ -91,6 +93,11 @@ impl TargetIsa for Isa {
         registers::INFO.clone()
     }
 
+    #[cfg(feature = "unwind")]
+    fn map_dwarf_register(&self, reg: RegUnit) -> Result<u16, RegisterMappingError> {
+        map_reg(self, reg).map(|r| r.0)
+    }
+
     fn encoding_info(&self) -> EncInfo {
         enc_tables::INFO.clone()
     }