[PowerPC] Only use PLT annotations if using PIC relocation model

Summary:

The default static (non-PIC, non-PIE) model for 32-bit powerpc does
not use @PLT annotations and relocations in GCC.  LLVM shouldn't use
@PLT annotations either, because it breaks secure-PLT linking with
(some versions of?) GNU LD.

Update the available-externally.ll test to reflect that default mode
should be the same as the static relocation, by using the same check
prefix.

Reviewed by:    sfertile
Differential Revision: https://reviews.llvm.org/D70570