Link shared libraries with --unresolved-symbols=import-dynamic. NFC

This is currently the default for shared libraries and PIC executables
in the linker but I'm hoping to change that soon.  This change means
emscripten will not get broken if/when that happens.

See emscripten-core#18198

tools/building.py

@@ -206,6 +206,7 @@ def lld_flags_for_executable(external_symbols):
 
   if settings.RELOCATABLE:
     cmd.append('--experimental-pic')
+    cmd.append('--unresolved-symbols=import-dynamic')
     if settings.SIDE_MODULE:
       cmd.append('-shared')
     else: