Use a different makefile variable for -D__wasilibc_use_wasip2 (#499)

This switches from `EXTRA_CFLAGS` to `CFLAGS` so when `EXTRA_CFLAGS` is
in the environment to specify `-g`, for example, it doesn't override
p2-handling logic.

Makefile

@@ -397,7 +397,7 @@ CFLAGS += -I$(LIBC_BOTTOM_HALF_CLOUDLIBC_SRC)
 endif
 
 ifeq ($(WASI_SNAPSHOT), p2)
-EXTRA_CFLAGS += -D__wasilibc_use_wasip2
+CFLAGS += -D__wasilibc_use_wasip2
 endif
 
 # Expose the public headers to the implementation. We use `-isystem` for