Revert "Reapply [runtimes] Fix building initial libunwind+libcxxabi+libcxx...

Revert "Reapply [runtimes] Fix building initial libunwind+libcxxabi+libcxx with compiler implied -lunwind"

This reverts commit 317dc31e.

After that change, OpenMP doesn't find dependencies in the host
system (it fails do find e.g. /usr/lib/x86_64-linux-gnu/libelf.so
which it found before), which causes some OpenMP target offloading
plugins to not be found. This doesn't break the build, but just
causes the AMDGPU OpenMP target plugin to be omitted. See
https://reviews.llvm.org/D113253#3181934 for the report of this
issue.