lang/guile[23]: prefer long over long long for scm_off_t

When long and long long are both the same size as off_t, prefer long
for scm_off_t even though it should make no difference, because llvm16
seems to consider them distinct types as arguments in function pointer
types. Also it seems cleaner.

fixes mailutils with GUILE=on on llvm16 on amd64.

Fixes: 94218d3a91189c7afea956e8905d5e5ecf879a2c
PR: 260960