[RISCV] Merge emitDirectiveOptionArchPlus and emitDirectiveOptionArchMinus...

[RISCV] Merge emitDirectiveOptionArchPlus and emitDirectiveOptionArchMinus into a single interface. NFC

Probably going to do some other refactors after this, but this one
was easy and clearly reduces duplicate code.

Reviewed By: StephenFan

Differential Revision: https://reviews.llvm.org/D151771