Commit 1: fix the bug: bigNatIndex# in the where
clause wasn't guarded by "bigNatIsZero a".
Commit 2: enabled -fcheck-prim-bounds in the validate flavour to make the test deterministically fail when the bug is present. It could be useful to detect other bugs too.
Commit 3: make the rts linker aware of rtsOutOfBoundsAccess
symbol