Stabilize Ratified RISC-V Target Features

As shortly discussed on Zulip
(https://rust-lang.zulipchat.com/#narrow/stream/250483-t-compiler.2Frisc-v/topic/Stabilization.20of.20RISC-V.20Target.20Features/near/394793704),
this commit stabilizes the ratified RISC-V instruction bases and
extensions.

Specifically, this commit stabilizes the:
* Atomic Instructions (A) on v2.0
* Compressed Instructions (C) on v2.0
* Integer Multiplication and Division (M) on v2.0
* Bit Manipulations (B) on v1.0 listed as `zba`, `zbc`, `zbs`
* Scalar Cryptography (Zk) v1.0.1 listed as `zk`, `zkn`, `zknd`, `zkne`, `zknh`, `zkr`, `zks`, `zksed`, `zksh`, `zkt`, `zbkb`, `zbkc` `zkbx`

compiler/rustc_codegen_ssa/src/target_features.rs

@@ -244,38 +244,38 @@ const MIPS_ALLOWED_FEATURES: &[(&str, Option<Symbol>)] = &[
 
 const RISCV_ALLOWED_FEATURES: &[(&str, Option<Symbol>)] = &[
     // tidy-alphabetical-start
-    ("a", Some(sym::riscv_target_feature)),
-    ("c", Some(sym::riscv_target_feature)),
+    ("a", None),
+    ("c", None),
     ("d", Some(sym::riscv_target_feature)),
     ("e", Some(sym::riscv_target_feature)),
     ("f", Some(sym::riscv_target_feature)),
-    ("m", Some(sym::riscv_target_feature)),
+    ("m", None),
     ("relax", Some(sym::riscv_target_feature)),
     ("unaligned-scalar-mem", Some(sym::riscv_target_feature)),
     ("v", Some(sym::riscv_target_feature)),
-    ("zba", Some(sym::riscv_target_feature)),
-    ("zbb", Some(sym::riscv_target_feature)),
-    ("zbc", Some(sym::riscv_target_feature)),
-    ("zbkb", Some(sym::riscv_target_feature)),
-    ("zbkc", Some(sym::riscv_target_feature)),
-    ("zbkx", Some(sym::riscv_target_feature)),
-    ("zbs", Some(sym::riscv_target_feature)),
+    ("zba", None),
+    ("zbb", None),
+    ("zbc", None),
+    ("zbkb", None),
+    ("zbkc", None),
+    ("zbkx", None),
+    ("zbs", None),
     ("zdinx", Some(sym::riscv_target_feature)),
     ("zfh", Some(sym::riscv_target_feature)),
     ("zfhmin", Some(sym::riscv_target_feature)),
     ("zfinx", Some(sym::riscv_target_feature)),
     ("zhinx", Some(sym::riscv_target_feature)),
     ("zhinxmin", Some(sym::riscv_target_feature)),
-    ("zk", Some(sym::riscv_target_feature)),
-    ("zkn", Some(sym::riscv_target_feature)),
-    ("zknd", Some(sym::riscv_target_feature)),
-    ("zkne", Some(sym::riscv_target_feature)),
-    ("zknh", Some(sym::riscv_target_feature)),
-    ("zkr", Some(sym::riscv_target_feature)),
-    ("zks", Some(sym::riscv_target_feature)),
-    ("zksed", Some(sym::riscv_target_feature)),
-    ("zksh", Some(sym::riscv_target_feature)),
-    ("zkt", Some(sym::riscv_target_feature)),
+    ("zk", None),
+    ("zkn", None),
+    ("zknd", None),
+    ("zkne", None),
+    ("zknh", None),
+    ("zkr", None),
+    ("zks", None),
+    ("zksed", None),
+    ("zksh", None),
+    ("zkt", None),
     // tidy-alphabetical-end
 ];