Link shared libraries with --unresolved-symbols=import-dynamic. NFC (#20795)

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 #18198

Author: sbc100

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: