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

The below culprit patch enabled the generation of asynchronous unwind
tables (-funwind-tables=2) by default for RISCV for both linux and
RISCVToolChain baremetal object. However, since there are 2 baremetal
toolchain objects for RISCV, this created a discrepancy between their
behavior. Moreover, enabling the generation of asynchronous unwind
tables based on whether `-gcc-toolchain` option is present or not
doesn't seem to be the best criteria to decide on the same. This patch
make the behavior consistent by disabling the unwind tables in
RISCVToolChain Baremetal object.

Culprit Patch - https://reviews.llvm.org/D145164