[llvm] [RISCV] Fix stack size computation when M extension disabled (PR #78602)

[Alex Bradbury via llvm-commits]

asb wrote:

@simeonkr do you have a core where this is needed? Although the RISC-V ISA allows vector extensions without M, I'm wondering if that case is worth supporting in LLVM.

https://github.com/llvm/llvm-project/pull/78602