[BOLT][RISCV] Implement getCalleeSavedRegs (#69161)

The main reason for implementing this now is to ensure the
`assume=abi.test` test passes on RISC-V. Since it uses
`--indirect-call-promotion=all`, it requires some support for register
analysis on the target.

Further testing and implementation of register/frame analysis on RISC-V
will come later.