Parallelize managed unit precompile

scripts/compile_wasm_unit

@@ -46,22 +46,29 @@ HEADER_HASH=$(find src/lib -maxdepth 1 -name '*.h' -type f -print0 | sort -z | x
 FLAGS_HASH=$(printf '%s\0' "${COMMON_FLAGS[@]}" -Isrc/lib | sha1sum | cut -c1-16)
 PCH_KEY=$(printf '%s\n%s\n%s\n%s\n' "$ABI_VERSION" "$TOOLCHAIN_ID" "$HEADER_HASH" "$FLAGS_HASH" | sha1sum | cut -c1-16)
 PCH_FN="$PCH_DIR/uce_lib-wasm-unit-$PCH_KEY.pch"
+PCH_TMP="$PCH_FN.tmp.$$"
 
 mkdir -p "$DEST_DIR" >/dev/null 2>&1
-trap 'rm -f "$OBJ_FN" "$ABI_TMP" "$MODULE_TMP" "$WASM_TMP" "$MAP_TMP"' EXIT
+trap 'rm -f "$OBJ_FN" "$ABI_TMP" "$MODULE_TMP" "$WASM_TMP" "$MAP_TMP" "$PCH_TMP"' EXIT
 
 build_pch_if_needed() {
 	if [ "$PCH_ENABLED" = "0" ]; then
 		return 0
 	fi
 	mkdir -p "$PCH_DIR"
+	exec 9>"$PCH_FN.lock"
+	flock 9
 	if [ -s "$PCH_FN" ] && [ -z "$(find src/lib -maxdepth 1 -name '*.h' -type f -newer "$PCH_FN" -print -quit)" ]; then
+		flock -u 9
+		exec 9>&-
 		return 0
 	fi
 	"$SDK/bin/clang++" "${COMMON_FLAGS[@]}" \
 		-Isrc/lib \
-		-x c++-header src/lib/uce_lib.h -o "$PCH_FN.tmp"
-	mv "$PCH_FN.tmp" "$PCH_FN"
+		-x c++-header src/lib/uce_lib.h -o "$PCH_TMP"
+	mv "$PCH_TMP" "$PCH_FN"
+	flock -u 9
+	exec 9>&-
 }
 
 cat > "$ABI_TMP" <<EOF