(More robustly) Fix watchOS extension linking when targeting versions less than 6.0.
(More robustly) Fix watchOS extension linking when targeting versions less than 6.0.

This is tricky to test because the error only manifests at runtime, but `llvm-objdump` can let us see the difference between a correct executable and a bad one. When the legacy library is correctly linked, `_WKExtensionMain` appears in the symbol table like this, defined at an actual address:

   000d5dc7 g     F __TEXT,__text _WKExtensionMain

When the legacy library is incorrectly linked, we see this undefined symbol instead:

   00000000         *UND* _WKExtensionMain

RELNOTES: None.
PiperOrigin-RevId: 281392737

bazelbuild/rules_apple,bazelbuild/rules_apple,bazelbuild/rules_apple,bazelbuild/rules_apple