AVL build isn't picking up general $CFLAGS

In the illumos-extra build, we set "$CFLAGS += -g" so that we can later ctfconvert the results and get CTF for mdb_v8.so itself.

However, this isn't passed down to the libavl build (CFLAGS is set directly to -fPIC $CFLAGS_ARCH). I can work around this by setting $CFLAGS_ARCH as well, but it would be nice if $CFLAGS was universally applied.