RISC-V: remove toolchain version checks for Zicbom

RISC-V: remove toolchain version checks for Zicbom

Commit b8c86872d1dc ("riscv: fix detection of toolchain Zicbom
support") fixed building on systems where Zicbom was supported by the
compiler/assembler but not by the linker in an easily backportable
manner.
Now that the we have insn-defs for the 3 instructions, toolchain support
is no longer required for Zicbom.
Stop emitting "_zicbom" in -march when Zicbom is enabled & drop the
version checks entirely.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20230108163356.3063839-4-conor@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>