[test] Add explicit dso_local to function declarations in static relocation model tests

They are currently implicit because TargetMachine::shouldAssumeDSOLocal implies
dso_local.

For such function declarations, clang -fno-pic emits the dso_local specifier.
Adding explicit dso_local makes these tests align with the clang behavior and
helps implementing an option to use GOT indirection when taking the address of a
function symbol in -fno-pic (to avoid a canonical PLT entry (SHN_UNDEF with
non-zero st_value)).