Rollup merge of #161064 - beetrees:revert-156188-riscv-target-feature-cfg-stable-toggle-unstable, r=workingjubilee

Revert "riscv: promote d, e, and f target_features to CfgStableToggleUnstable"

Reverts rust-lang/rust#156188 per https://github.com/rust-lang/rust/issues/150257#issuecomment-5268676292.

This is my first time doing this kind of revert, let me know if I've done anything wrong.

r? lang

@rustbot label: +beta-nominated