Fixes #984: Add a isa::lookup_by_name function;

This removes the explicit dependency on target-lexicon for the embedder,
which can instead use the ISA's name directly. It can simplify
dependency management, in particular avoid the need for synchronizing
the target-lexicon dependencies versions.

It also tweak the error when an ISA isn't built as part of Cranelift to
be a SupportDisabled error; this was dead code before this.

cranelift/codegen/src/isa/mod.rs

@@ -66,7 +66,7 @@ use crate::timing;
 use core::fmt;
 use failure_derive::Fail;
 use std::boxed::Box;
-use target_lexicon::{Architecture, PointerWidth, Triple};
+use target_lexicon::{triple, Architecture, PointerWidth, Triple};
 
 #[cfg(feature = "riscv")]
 mod riscv;
@@ -97,13 +97,13 @@ macro_rules! isa_builder {
         };
         #[cfg(not(feature = $feature))]
         fn $name(_triple: Triple) -> Result<Builder, LookupError> {
-            Err(LookupError::Unsupported)
+            Err(LookupError::SupportDisabled)
         }
         $name
     }};
 }
 
-/// Look for a supported ISA with the given `name`.
+/// Look for an ISA for the given `triple`.
 /// Return a builder that can create a corresponding `TargetIsa`.
 pub fn lookup(triple: Triple) -> Result<Builder, LookupError> {
     match triple.architecture {
@@ -117,6 +117,13 @@ pub fn lookup(triple: Triple) -> Result<Builder, LookupError> {
     }
 }
 
+/// Look for a supported ISA with the given `name`.
+/// Return a builder that can create a corresponding `TargetIsa`.
+pub fn lookup_by_name(name: &str) -> Result<Builder, LookupError> {
+    use std::str::FromStr;
+    lookup(triple!(name))
+}
+
 /// Describes reason for target lookup failure
 #[derive(Fail, PartialEq, Eq, Copy, Clone, Debug)]
 pub enum LookupError {