[PowerPC] Emit dcbt and dcbtst in place of their extended mnemonics on AIX

On AIX, the system assembler does not support the extended mnemonics
dcbtt and dcbtstt. This patch stops them from being emitted on
AIX and emits the base mnemonics instead, dcbt X, X, 16 and
dcbtstt X, X, 16 respectively.

Differential revision: https://reviews.llvm.org/D111258