equal hash code: avoid dropping useful bits

Closes PR 14059

Symbols in the PR were mapped to coliding hashes in
groups of 4 because the low 2 bits of the `eq?` hash
code were begin dropped to generate an `equal?` hash
code. Those two bits got lost due to a refectoring
a while back that moved the dropping of two useless
bits to a more centralized place, but the 2-bit shift
did not get removed from the `equal` hash code comparision.

The PR's example program will still generate groups of 2
when hashing around 10k symbols (which used to be groups of 8).
That's because there's a bit in the hash-code counter that
turns out to be forced to 1.
further tighten eq-hash code generation

Avoids including the bit thet indicates whether the object
is GCable in the eq hash code (in a configuration where
bits are available in the GC header for hashing).