Auto merge of #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`

compiler/rustc_span/src/symbol.rs

@@ -327,6 +327,9 @@ symbols! {
         as_ptr,
         as_str,
         asm,
+        asm_const,
+        asm_experimental_arch,
+        asm_sym,
         assert,
         assert_inhabited,
         assert_macro,