rustc: Modernize wasm checks for atomics

This commit modernizes how rustc checks for whether the `atomics`
feature is enabled for the wasm target. The `sess.target_features` set
is consulted instead of fiddling around with dealing with various
aspects of LLVM and that syntax.

src/librustc_codegen_llvm/back/write.rs

@@ -23,6 +23,7 @@ use rustc_middle::bug;
 use rustc_middle::ty::TyCtxt;
 use rustc_session::config::{self, Lto, OutputType, Passes, SanitizerSet, SwitchWithOptPath};
 use rustc_session::Session;
+use rustc_span::symbol::sym;
 use rustc_span::InnerSpan;
 use rustc_target::spec::{CodeModel, RelocModel};
 
@@ -140,7 +141,7 @@ pub fn target_machine_factory(
     // lower atomic operations to single-threaded operations.
     if singlethread
         && sess.target.target.llvm_target.contains("wasm32")
-        && features.iter().any(|s| *s == "+atomics")
+        && sess.target_features.contains(&sym::atomics)
     {
         singlethread = false;
     }

src/librustc_codegen_ssa/back/linker.rs

@@ -1,6 +1,7 @@
 use super::archive;
 use super::command::Command;
 use super::symbol_export;
+use rustc_span::symbol::sym;
 
 use std::ffi::{OsStr, OsString};
 use std::fs::{self, File};
@@ -1036,9 +1037,7 @@ impl<'a> WasmLd<'a> {
         //
         // * `--export=*tls*` - when `#[thread_local]` symbols are used these
         //   symbols are how the TLS segments are initialized and configured.
-        let atomics = sess.opts.cg.target_feature.contains("+atomics")
-            || sess.target.target.options.features.contains("+atomics");
-        if atomics {
+        if sess.target_features.contains(&sym::atomics) {
             cmd.arg("--shared-memory");
             cmd.arg("--max-memory=1073741824");
             cmd.arg("--import-memory");

src/librustc_span/symbol.rs

@@ -159,6 +159,7 @@ symbols! {
         assume_init,
         async_await,
         async_closure,
+        atomics,
         attr,
         attributes,
         attr_literals,