riscv: show help string for riscv-specific targets

Define the archhelp variable so that 'make ACRH=riscv help' will show
the targets specific to building a RISC-V kernel like other
architectures.

Tested-by: Björn Töpel <bjorn@rivosinc.com>
Signed-off-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Reviewed-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20240504193446.196886-3-emil.renner.berthing@canonical.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>