Add multithread support to custom broadcast

Author: lorenzovarese

src/embedded/cust_broadcast.jl

@@ -260,7 +260,7 @@ end
 
     # Performance may vary depending on whether `@inbounds` is placed outside the
     # for loop or not. (cf. https://github.com/JuliaLang/julia/issues/38086)
-    @inbounds @simd for I in eachindex(dest)
+    @inbounds @threads for I in eachindex(dest)
         dest[I] = bc′[I]
     end
     return dest