[libunwind] [RISCV] Allow libunwind to build for rv32e (PR #98855)

[Sam Elliott via cfe-commits]

lenary wrote:

@ArcaneNibble your contributions now definitely meet the threshold for commit access - please request it following the instructions in the Developer Policy: https://llvm.org/docs/DeveloperPolicy.html#obtaining-commit-access (also take a read of the rest of the policy to understand how the processes are expected to work once you have it)

https://github.com/llvm/llvm-project/pull/98855