makefile: change name of clang-8 executable to clang-9

Author: Pat Hickey

Makefile

@@ -137,7 +137,7 @@ build/config.BUILT:
 build: build/llvm.BUILT build/wasi-libc.BUILT build/compiler-rt.BUILT build/libcxxabi.BUILT build/libcxx.BUILT build/config.BUILT
 
 strip: build/llvm.BUILT
-	cd $(PREFIX)/bin; strip clang-8 llc lld llvm-ar
+	cd $(PREFIX)/bin; strip clang-9 llc lld llvm-ar
 
 package: build/package.BUILT