Rollup merge of #144116 - nikic:llvm-21-fixes, r=dianqk

Fixes for LLVM 21

This fixes compatibility issues with LLVM 21 without performing the actual upgrade. Split out from rust-lang/rust#143684.

This fixes three issues:
 * Updates the AMDGPU data layout for address space 8.
 * Makes emit-arity-indicator.rs a no_core test, so it doesn't fail on non-x86 hosts.
 * Explicitly sets the exception model for wasm, as this is no longer implied by `-wasm-enable-eh`.

Author: matthiaskrgr