Explicitly fail if multicore is enabled for wasm32 without atomics

`maybe-rayon` has this case fall back to not using rayon, which while
correct is potentially confusingly non-performant. Developers building
for WASM will have similar things they need to handle themselves; they
can conditionally enable `multicore` there.

halo2_proofs/src/multicore.rs

@@ -2,6 +2,15 @@
 //! `halo2`. It's currently just a (very!) thin wrapper around `rayon` but may
 //! be extended in the future to allow for various parallelism strategies.
 
+#[cfg(all(
+    feature = "multicore",
+    target_arch = "wasm32",
+    not(target_feature = "atomics")
+))]
+compile_error!(
+    "The multicore feature flag is not supported on wasm32 architectures without atomics"
+);
+
 pub use maybe_rayon::{
     iter::{IntoParallelIterator, ParallelIterator},
     join, scope, Scope,