upgrade to target-lexicon 0.8.0

* the target-lexicon crate no longer has or needs the std feature
  in cargo, so we can delete all default-features=false, any mentions
  of its std feature, and the nostd configs in many lib.rs files
* the representation of arm architectures has changed, so some case
  statements needed refactoring

cranelift/codegen/src/isa/arm32/mod.rs

@@ -42,14 +42,13 @@ fn isa_constructor(
     builder: shared_settings::Builder,
 ) -> Box<dyn TargetIsa> {
     let level1 = match triple.architecture {
-        Architecture::Thumbv6m | Architecture::Thumbv7em | Architecture::Thumbv7m => {
-            &enc_tables::LEVEL1_T32[..]
+        Architecture::Arm(arm) => {
+            if arm.is_thumb() {
+                &enc_tables::LEVEL1_T32[..]
+            } else {
+                &enc_tables::LEVEL1_A32[..]
+            }
         }
-        Architecture::Arm
-        | Architecture::Armv4t
-        | Architecture::Armv5te
-        | Architecture::Armv7
-        | Architecture::Armv7s => &enc_tables::LEVEL1_A32[..],
         _ => panic!(),
     };
     Box::new(Isa {

cranelift/codegen/src/isa/call_conv.rs

@@ -31,6 +31,7 @@ impl CallConv {
             // uses System V.
             Ok(CallingConvention::SystemV) | Err(()) => CallConv::SystemV,
             Ok(CallingConvention::WindowsFastcall) => CallConv::WindowsFastcall,
+            Ok(unimp) => unimplemented!("calling convention: {:?}", unimp),
         }
     }
 

cranelift/codegen/src/isa/mod.rs

@@ -111,15 +111,8 @@ pub fn lookup(triple: Triple) -> Result<Builder, LookupError> {
         Architecture::I386 | Architecture::I586 | Architecture::I686 | Architecture::X86_64 => {
             isa_builder!(x86, "x86")(triple)
         }
-        Architecture::Thumbv6m
-        | Architecture::Thumbv7em
-        | Architecture::Thumbv7m
-        | Architecture::Arm
-        | Architecture::Armv4t
-        | Architecture::Armv5te
-        | Architecture::Armv7
-        | Architecture::Armv7s => isa_builder!(arm32, "arm32")(triple),
-        Architecture::Aarch64 => isa_builder!(arm64, "arm64")(triple),
+        Architecture::Arm { .. } => isa_builder!(arm32, "arm32")(triple),
+        Architecture::Aarch64 { .. } => isa_builder!(arm64, "arm64")(triple),
         _ => Err(LookupError::Unsupported),
     }
 }