[clang] [RISCV] Disable generation of asynchronous unwind tables for RISCV baremetal (PR #81727)

[Alex Bradbury via cfe-commits]

asb wrote:

CC @petrhosek too who I know [hopes to merge the RISCVToolChain and BareMetal toolchains](https://discourse.llvm.org/t/merging-riscvtoolchain-and-baremetal-toolchains/75524). Matching the GCC behaviour seems sensible to me, and I'm hopeful that the planned future refactoring will avoid this kind of discrepancy in behaviour based on `--gcc-toolchain` presence in the future.

https://github.com/llvm/llvm-project/pull/81727