build-libcxx: Stop manually specifying -lunwind

Specifying -lunwind isn't needed if building a version of libcxx
after 5f9be2c3e37c0428ba56876dd84af04b8d9d8915 (Feb 2021); then
libcxx is built with -nostdlib++ which keeps automatically linking
in -lunwind if -unwindlib=libunwind is used.

build-libcxx.sh

@@ -147,7 +147,6 @@ build_all() {
             -DLIBCXX_CXX_ABI_LIBRARY_PATH=../../libcxxabi/build-$arch-$type/lib \
             -DLIBCXX_LIBDIR_SUFFIX="" \
             -DLIBCXX_INCLUDE_TESTS=FALSE \
-            -DCMAKE_SHARED_LINKER_FLAGS="-lunwind" \
             -DLIBCXX_ENABLE_ABI_LINKER_SCRIPT=FALSE \
             ..
         $BUILDCMD ${CORES+-j$CORES} generate-cxx-headers