Auto merge of rust-lang#90348 - Amanieu:asm_feature_gates, r=joshtriplett

Add features gates for experimental asm features

This PR splits off parts of `asm!` into separate features because they are not ready for stabilization.

Specifically this adds:
- `asm_const` for `const` operands.
- `asm_sym` for `sym` operands.
- `asm_experimental_arch` for architectures other than x86, x86_64, arm, aarch64 and riscv.

r? `@nagisa`

Author: bors

core/src/lib.rs

@@ -193,6 +193,7 @@
 #![feature(try_blocks)]
 #![feature(unboxed_closures)]
 #![feature(unsized_fn_params)]
+#![cfg_attr(not(bootstrap), feature(asm_const))]
 //
 // Target features:
 #![feature(aarch64_target_feature)]