[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