fix errors/warnings from the stabilization of cfg_target_feature and target_feature (#432)

* fix build after stabilization of cfg_target_feature and target_feature

* fix doc tests

* fix spurious unused_attributes warning

* fix more unused attribute warnings

* More unnecessary target features

* Remove no longer needed trait imports

* Remove fixed upstream workarounds

* Fix parsing the #[assert_instr] macro

Following upstream proc_macro changes

* Fix form and parsing of #[simd_test]

* Don't use Cargo features for testing modes

Instead use RUSTFLAGS with `--cfg`. This'll help us be compatible with the
latest Cargo where a tweak to workspaces and features made the previous
invocations we had invalid.

* Don't thread RUSTFLAGS through docker

* Re-gate on x86 verification

Closes #411

library/stdarch/crates/assert-instr-macro/src/lib.rs

@@ -137,7 +137,7 @@ struct Invoc {
 }
 
 impl syn::synom::Synom for Invoc {
-    named!(parse -> Self, map!(parens!(do_parse!(
+    named!(parse -> Self, do_parse!(
         instr: syn!(syn::Expr) >>
         args: many0!(do_parse!(
             syn!(syn::token::Comma) >>
@@ -150,7 +150,7 @@ impl syn::synom::Synom for Invoc {
             instr,
             args,
         })
-    )), |p| p.1));
+    ));
 }
 
 struct Append<T>(T);