Move the allocation of CAF blackholes into 'newCAF' (#8590)

We now do the allocation of the blackhole indirection closure inside the
RTS procedure 'newCAF' instead of generating the allocation code inline
in the closure body of each CAF.  This slightly decreases code size in
modules with a lot of CAFs.

As a result of this change, for example, the size of DynFlags.o drops by
~60KB and HsExpr.o by ~100KB.
Update and deduplicate the comments on CAF management (#8590)