Fix argument and return value alignment in `scm_i_foreign_call'.

Fix argument and return value alignment in `scm_i_foreign_call'.

* libguile/foreign.c (scm_i_foreign_call): Fix the computation of
  ARG_SIZE wrt. alignment.  Arrange so that the address ARGS[i] is
  aligned, rather than checking whether OFF is aligned.  Have the RVALUE
  be at least word-aligned, which fixes calls to `char'-returning
  functions on `armv5tel-*-linux-gnueabi'.