Re-enable compat build for libc and libutil

I don't remember why it was originally disabled, but now I can't think of
a good reason to deviate from unmodified sources.  The compat parts of
the libs are useless for rumprun, but due to .a semantics they'll just
not be linked into the final image, i.e. mostly harmless.
Make users of libc provide __libc_static_tls_init()

We might move to the one in libc/tls some day ... but not today.
In any case, this approach gets rid of one more local diff.
No need to disable strict-protos here for ucontext

We already build libc without -Werror because of other lossage :(