[RISCV] Make Zbc imply Zbkc. (#185543)
Zbkc contains 2 of the 3 instructions from Zbc. Making Zbc imply Zbkc
will make the __riscv_zbkc define be set when Zbc is enabled.
This does not change the diagnostics printed by the assembler.
There's a PR to add this rule to the ISA manual too
https://github.com/riscv/riscv-isa-manual/pull/2524
