Relax witness assertions related to prof_gdump().

In some cases the prof machinery allocates (in order to modify the
bt2gctx hash table), and such operations are synchronized via
bt2gctx_mtx.  Rather than asserting that no locks are held on entry
into functions that may call prof_gdump(), make the weaker assertion
that no "core" locks are held.  The prof machinery enqueues dumps
triggered by prof_gdump() calls when bt2gctx_mtx is held, so this
weakened assertion avoids false failures in such cases.