[llvm] [RISCV][MRI] Account for fixed registers when determining callee saved regs (PR #115756)

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

The behavior differs from GCC for AArch64, ARM, Hexagon, SPARC, M68k and RISC-V. 

In LLVM, different targets have different interfaces to check whether the register was `-ffixed-XXX`. I am proposing that each target migrate to this common interface in follow up patches.

This patch adds the interface, and migrates RISC-V and M68k to that interface, since they are already using the interface.

https://github.com/llvm/llvm-project/pull/115756