[ELF] Fix 3 tests after D56828

arm-plt-reloc.s arm-thumb-plt-reloc.s: update offset calculations

pack-dyn-relocs-loop.s: this test is very sensitive to exact section
offsets and sizes. If we comment out the following two lines in
SyntheticSections.cpp, we should reproduce `ld.lld: error: thunk
creation not converged` caused by oscillation of the section size.

  if (RelocData.size() < OldSize)
    RelocData.append(OldSize - RelocData.size(), 0);

Use -z norelro to counteract the layout change (to be more specific,
we have to place .dynamic below .foo so that offset(foo) remains 0x10004)

llvm-svn: 356229