separate-debug-info.sh: succeed when output does not contain elf files

Currently, separate-debug-info adds a debug output, and the build fail when it is
not created. the output is only created when at least one elf file is
stripped.
As a result, adding separateDebugInfo = true on a lib will break the
static build (unless the lib also contains an executable). In order to
not have to remember to add an exception every time, let's just create
the debug output unconditionally.

pkgs/build-support/setup-hooks/separate-debug-info.sh

@@ -11,6 +11,9 @@ _separateDebugInfo() {
     local dst="${debug:-$out}"
     if [ "$prefix" = "$dst" ]; then return 0; fi
 
+    # in case there is nothing to strip, don't fail the build
+    mkdir -p "$dst"
+
     dst="$dst/lib/debug/.build-id"
 
     # Find executables and dynamic libraries.