rustc_target: Adjust RISC-V feature implication (Za64rs and Za128rs)

The Za64rs extension (reservation set -- a primitive memory unit of LR/SC
atomic operations -- is naturally aligned and *at most* 64 bytes) is a
superset of the Za128rs extension (*at most* 128 bytes; note that smaller
the reservation set is, more fine grained control over atomics).

This commit handles this as a feature implication.

Author: a4lg

compiler/rustc_target/src/target_features.rs

@@ -507,7 +507,7 @@ static RISCV_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
     ("unaligned-vector-mem", Unstable(sym::riscv_target_feature), &[]),
     ("v", Unstable(sym::riscv_target_feature), &["zvl128b", "zve64d"]),
     ("za128rs", Unstable(sym::riscv_target_feature), &[]),
-    ("za64rs", Unstable(sym::riscv_target_feature), &[]),
+    ("za64rs", Unstable(sym::riscv_target_feature), &["za128rs"]), // Za64rs ⊃ Za128rs
     ("zaamo", Unstable(sym::riscv_target_feature), &[]),
     ("zabha", Unstable(sym::riscv_target_feature), &["zaamo"]),
     ("zacas", Unstable(sym::riscv_target_feature), &["zaamo"]),