Revert r307689

The proposed change ("Fix building of llvm's unwind if gcc has been
also built") breaks the build with clang/llvm.

Tested with...

(
export SRCCONF=/dev/null WITH_CLANG=
cd gnu/lib/libgcc; make obj; make depend; make all
)

MFC after: 3 days
X-MFC with: r307689
Pointyhat to: bapt
Reported by: Jenkins, O. Hartmann <ohartman@zedat.fu-berlin.de>
Sponsored by: Dell EMC Isilon