integer-gmp: Fix #11296

This was introduced by a mental fumble in
9e8562ae wherein I replaced
`getSizeofMutableByteArray` with `getSizeofMutBigNat`. This was noticed
by invalid integers being produced on 32-bit machines in #11296.

Test Plan: Validate

Reviewers: hvr, goldfire, austin

Reviewed By: hvr

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1707

GHC Trac Issues: #11296