compiler: fix out-of-bound memory access of genSym on 32-bit

This commit fixes an unnoticed out-of-bound memory access of genSym on
32-bit. ghc_unique_inc is 32-bit sized/aligned on 32-bit platforms,
but we mistakenly treat it as a Word64 pointer in genSym, and
therefore will accidentally load 2 garbage higher bytes, or with a
small but non-zero chance, overwrite something else in the data
section depends on how the linker places the data segments. This
regression was introduced in !11802 and fixed here.