[PowerPC] The VK_PLT symbolref modifier is only used on 32-bit ELF. [NFC]

Fix a conditional that guarded code for execution only on 32-bit ELF by
checking that the Subtarget was not 64-bit and not-Darwin. By adding a new
target ABI (AIX), the condition is no longer correct. This code is dead for
AIX, due to a 'report_fatal_error' for thread local storage usage earlier in the
pipeline, but needs to be modifed as part of Darwins removal from the
PowerPC backend.