Rollup merge of #150073 - daxpedda:internal-unstable-asm-experimental-arch, r=nnethercote

Make `asm_experimental_arch` work in `allow_internal_unstable` macros

This change makes it possible to use unstable `asm!`, usually requiring `feature(asm_experimental_arch)`, in proc-macros with the `allow_internal_unstable` attribute.

The test was added on a target where `asm!` is unstable: Wasm. However, this affects *any* target with an unstable `asm!` implementation.