Pass -O2 -DNDEBUG to wasi-libc again (WebAssembly#423)

I forgot in WebAssembly#422 that by specifying `EXTRA_CFLAGS` to the wasi-libc
build that it would override the defaults of wasi-libc which is to pass
these two flags in. This passes them back in to ensure the default build
still has the same flags.

Makefile

@@ -143,7 +143,7 @@ WASI_LIBC_MAKEFLAGS = \
 	AR=$(BUILD_PREFIX)/bin/llvm-ar \
 	NM=$(BUILD_PREFIX)/bin/llvm-nm \
 	SYSROOT=$(BUILD_PREFIX)/share/wasi-sysroot \
-	EXTRA_CFLAGS="$(WASI_SDK_CFLAGS)" \
+	EXTRA_CFLAGS="$(WASI_SDK_CFLAGS) -O2 -DNDEBUG" \
 	TARGET_TRIPLE=$(1)
 
 build/wasi-libc.BUILT: build/compiler-rt.BUILT build/wasm-component-ld.BUILT