Use -unwindlib=libunwind instead of merging libunwind into libc++

This makes the build of libcxx a bit cleaner, and simplifies later
building and testing of libcxx.

build-llvm.sh

@@ -157,6 +157,7 @@ if [ -n "$HOST" ]; then
     # the frontend wrappers, but this makes sure they are enabled by
     # default if that wrapper is bypassed as well.
     CMAKEFLAGS="$CMAKEFLAGS -DCLANG_DEFAULT_RTLIB=compiler-rt"
+    CMAKEFLAGS="$CMAKEFLAGS -DCLANG_DEFAULT_UNWINDLIB=libunwind"
     CMAKEFLAGS="$CMAKEFLAGS -DCLANG_DEFAULT_CXX_STDLIB=libc++"
     CMAKEFLAGS="$CMAKEFLAGS -DCLANG_DEFAULT_LINKER=lld"
     BUILDDIR=$BUILDDIR-$HOST