Minor cleanup private_builtin.m.

library/private_builtin.m:
    Remove two long-unneeded predicates, trailed_nondet_pragma_foreign_code
    and reclaim_heap_nondet_pragma_foreign_code.

    Move foreign_code pragmas together into a single group at the end of the
    module.

    Fix white space.

library/builtin.m:
    Fix white space.