`symgen exports` replaces -export_dynamic on Apple and curates
Android's dynamic symbols with a version script. `symgen stub` then
emits sdk/stub-{macos,ios,tvos}-<arch>, sdk/stub-android-<arch>.so,
and sdk/windows-<arch>.lib.
Apple: mods are now MH_BUNDLE (.so) linked with -bundle_loader,
replacing the deprecated (on iOS) -undefined dynamic_lookup.
Windows: clang-cl mods link dusklight.exe directly; lld's mingw driver
synthesizes imports from the export table. cl still requires the
import library.
Luke Street