[PowerPC][test] Add explicit dso_local to definitions in ELF static relocation model tests

TargetMachine::shouldAssumeDSOLocal currently implies dso_local for such definitions.

Adding explicit dso_local makes these tests align with the clang -fpic behavior
and allow the removal of the TargetMachine::shouldAssumeDSOLocal special case.

Rewrite preemption.ll to dsolocal-static.ll and dsolocal-pic.ll, and add
"PIC Level" metadata.