[RISCV] Disallow target attribute use in multiversioning (#85899)

For RISC-V target only `target_clones` and `target_version` can enable
function multiversion(FMV).

This patch make target attribute trigger redefinition instead of emit
FMV.

Here is spec
https://github.com/riscv-non-isa/riscv-c-api-doc/blob/master/riscv-c-api.md#__attribute__targetattr-string