[clang] Change CodeGenOptions::RelaxELFRelocations/assembler defaults to match MC default

MC default was flipped in 2016.
CMake ENABLE_X86_RELAX_RELOCATIONS defaults to on in 2020 (c41a18cf).
It makes sense for the CodeGenOptions::RelaxELFRelocations to match, so
that most -cc1/-cc1as command lines won't have this option.

This also fixes a minor issue: -fno-plt -S will now use GOT for
__tls_get_addr calls, matching -fno-plt -c.