Add valid features (like bulk-memory and mutable-globals) to prevent cargo warnings

tools and infrastructure cargo
1

cargo shows the following message while building std using RUSTFLAGS with custom target features:

warning: unknown feature specified for `-Ctarget-feature`: `bulk-memory`
  |
  = note: it is still passed through to the codegen backend
  = note: consider filing a feature request

warning: unknown feature specified for `-Ctarget-feature`: `mutable-globals`
  |
  = note: it is still passed through to the codegen backend
  = note: consider filing a feature request

Since these are valid features, these warnings should not be present.

2

Those warnings are emitted by rustc. Adding these features to rust/target_features.rs at e6327bc8b8d437b66ff91d9ce798a9eb45310967 · rust-lang/rust · GitHub should fix the warnings.