[test] Add explicit dso_local to constant/global variable declarations

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

For external data, clang -fno-pic emits the dso_local specifier for ELF and
non-MinGW COFF. Adding explicit dso_local makes these tests in align with the
clang behavior and helps implementing an option to use GOT indirection for
external data access in -fno-pic mode (to avoid copy relocations).