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/fde.rs

@@ -2,6 +2,7 @@
 
 use crate::binemit::{FrameUnwindOffset, FrameUnwindSink, Reloc};
 use crate::ir::{FrameLayoutChange, Function};
+use crate::isa::fde::RegisterMappingError;
 use crate::isa::{CallConv, RegUnit, TargetIsa};
 use alloc::vec::Vec;
 use core::convert::TryInto;
@@ -10,7 +11,6 @@ use gimli::write::{
     FrameDescriptionEntry, FrameTable, Result, Writer,
 };
 use gimli::{Encoding, Format, LittleEndian, Register, X86_64};
-use thiserror::Error;
 
 pub type FDERelocEntry = (FrameUnwindOffset, Reloc);
 
@@ -137,16 +137,6 @@ pub fn map_reg(
     }
 }
 
-#[derive(Error, Debug)]
-pub enum RegisterMappingError {
-    #[error("unable to find bank for register info")]
-    MissingBank,
-    #[error("register mapping is currently only implemented for x86_64")]
-    UnsupportedArchitecture,
-    #[error("unsupported register bank: {0}")]
-    UnsupportedRegisterBank(&'static str),
-}
-
 fn to_cfi(
     isa: &dyn TargetIsa,
     change: &FrameLayoutChange,