Rollup merge of #153857 - RalfJung:cfg-abi, r=Mark-Simulacrum

Rename `target.abi` to `target.cfg_abi` and enum-ify llvm_abiname

See [Zulip](https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/De-spaghettifying.20ABI.20controls/with/578893542) for more context. Discussed a bit in rust-lang/rust#153769 (comment) too.

This renames `target.abi` to `target.cfg_abi` to make it less likely that someone will use it to determine things about the actual ccABI, i.e. the calling convention used on the target. `target.abi` does not control that calling convention, it just *sometimes* informs the user about that calling convention (and also about other aspects of the ABI).

Also turn llvm_abiname into an enum to make it more natural to match on.
Cc @workingjubilee @madsmtm

Author: JonathanBrouwer

src/lib.rs

@@ -46,7 +46,7 @@ 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::{Abi, Arch, Env, Os};
+use rustc_target::spec::{Arch, CfgAbi, Env, Os};
 
 pub use crate::config::*;
 use crate::prelude::*;
@@ -178,7 +178,7 @@ impl CodegenBackend for CraneliftCodegenBackend {
         let has_reliable_f16_f128 = !(sess.target.arch == Arch::X86_64
             && sess.target.os == Os::Windows
             && sess.target.env == Env::Gnu
-            && sess.target.abi != Abi::Llvm);
+            && sess.target.cfg_abi != CfgAbi::Llvm);
 
         // FIXME(f128): f128 math operations need f128 math symbols, which currently aren't always
         // filled in by compiler-builtins. The only libc that provides these currently is glibc.