Instead of ld -dc use ld -d, supported by both ld.bfd and ld.lld >= 15

Instead of ld -dc use ld -d, supported by both ld.bfd and ld.lld >= 15

Upstream LLVM decided to drop the -dc flag and keep its -d alias for
some time:
  https://github.com/llvm/llvm-project/commit/0688b00fc39fe87ec6dd3d37a732d067ff487f16

So this is only a workaround to let us update llvm to a recent release.
We'll probably need another fix not so distant future.  Another approach
would be to just drop -dc from the linker invocation, and use
cc -fno-common when building instbin with base-gcc (already the default
with base-clang).  But this approach needs more testing than I can
provide, as deraadt@ feared it would break in subtle ways.

ld -d compat alias pointed out by robert@, ok robert@