[ELF] Enforce two dashes for Flag options not supported by GNU ld (i.e. no compatibility burden)

Announced on https://lists.llvm.org/pipermail/llvm-dev/2020-May/141416.html

Similar to D79371, but for `multiclass B` (convenience helper for defining --foo and --no-foo)

Some changed options are also used by gold, but I haven't seen their
one-dash use cases outside of lld's testsuite.