cmd/dist: support HOST_CFLAGS/HOST_LDFLAGS for building 6g/6l/etc.

<pre>If you want to pass, say, -fstack-protector-all to the C compiler, then you can mostly
do it by wedging that in to the CC environment variable. However, that's a bit messy,
and there's no way to pass extra flags to the C linker.</pre>
