Bugfix for register spilling

[AdUhTkJm]

Bugfixes:

1. Label.t used to only compare stamps, ignoring label names. Now it takes both into account.
2. In riscv_reg_spill.ml, the global variable rpo is now properly initialized.
3. Vec.make ~dummy len actually makes a zero-length vector, which is counterintuitive. To avoid disturbing existing code, I replaced their calls in riscv_reg_spill.ml with Vec.of_list (List.init ...).