build: deduplicate Objective-C runtime linkage

[steipete]

Summary

• Remove redundant explicit -lobjc flags from Darwin cgo directives.
• Keep all existing Foundation, Virtualization, and Cocoa framework links unchanged.
• Rely on cgo/clang's Objective-C runtime linkage, which already contributes the required single runtime link.

This removes the Apple linker warning:

ld: warning: ignoring duplicate libraries: '-lobjc'

The warning was visible downstream because each Objective-C source file contributed another explicit -lobjc to the package link command.

Verification

• go test -run '^$' . ./internal/objc (clean compile; warning absent)
• go test -run '^$' ./...
• go vet ./...
• git diff --check