rustc_target: introduce Abi

Improve type safety by using an enum rather than strings.

Author: tamird

src/lib.rs

@@ -49,7 +49,7 @@ use rustc_middle::dep_graph::{WorkProduct, WorkProductId};
 use rustc_session::Session;
 use rustc_session::config::OutputFilenames;
 use rustc_span::{Symbol, sym};
-use rustc_target::spec::Arch;
+use rustc_target::spec::{Abi, Arch};
 
 pub use crate::config::*;
 use crate::prelude::*;
@@ -186,7 +186,7 @@ impl CodegenBackend for CraneliftCodegenBackend {
         let has_reliable_f16_f128 = !(sess.target.arch == Arch::X86_64
             && sess.target.os == "windows"
             && sess.target.env == "gnu"
-            && sess.target.abi != "llvm");
+            && sess.target.abi != Abi::Llvm);
 
         TargetConfig {
             target_features,