[clang][RISCV][Zicfilp] Force user to use `-mcf-branch-label-scheme=unlabeled` (#152122)

Expected Behavior:

When `-fcf-protection=branch|full` is specified, it's an error to omit
`-mcf-branch-label-scheme=unlabeled`.

Context:

When using forward-edge control flow integrity feature based on the
RISC-V Zicfilp extension, the `-mcf-branch-label-scheme` option selects
the encoding scheme used in the landing pad labels. The spec defines 2
schemes: `func-sig` and `unlabeled`, with the former specified as the
default. However the `func-sig` backend is still under active
development and won't land anytime soon; in the meanwhile, the
`unlabeled` scheme almost has complete support in the toolchain now.

Given that Clang currently accepts and defaults to
`-mcf-branch-label-scheme=func-sig` but doesn't work correctly, we want
to formally forbid the user from using
`-mcf-branch-label-scheme=func-sig` for now until the `func-sig` scheme
is properly supported. We choose to do so by forcing the user to specify
`-mcf-branch-label-scheme=unlabeled` (instead of changing the default to
`unlabeled`) so the default of `func-sig` can be retained according to
the spec and build scripts targeting the current Clang can be
forward-compatible with the Clang that supports the complete `func-sig`
scheme.