Add winarm64 flags to Rcpp

Author: andrjohns

R/tbb.R

@@ -49,8 +49,14 @@ tbbCxxFlags <- function() {
    flags <- character()
 
    # opt-in to TBB on Windows
-   if (is_windows())
+   if (is_windows()) {
       flags <- c(flags, "-DRCPP_PARALLEL_USE_TBB=1")
+      if (R.version$arch == "aarch64") {
+         # TBB does not have assembly code for Windows ARM64
+         # so we need to use compiler builtins
+         flags <- c(flags, "-DTBB_USE_GCC_BUILTINS")
+      }
+   }
 
    # if TBB_INC is set, apply those library paths
    tbbInc <- Sys.getenv("TBB_INC", unset = TBB_INC)