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

[Kito Cheng via cfe-commits]

kito-cheng wrote:

RISC-V GCC has enabled `-fasynchronous-unwind-tables` and `-funwind-tables` by default for Linux target, and disabled by default for baremetal, so generally LGTM since it align the behavior with GCC, but I would like to wait @asb's response. 

NOTE: The patch[1] is come from SUSE folks, so I assume it's necessary from the distro's point of view.

Ref:
[1] https://github.com/gcc-mirror/gcc/commit/3cd08f7168c196d7a481b9ed9f4289fd1f14eea8

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