Arbitrary code execution during build on darwin via "-l to_library" in cgo in cmd/go

On Darwin, building a malicious package that uses cgo could cause arbitrary code execution when using the Apple linker due to passing "-l to_library <path>" in a "#cgo LDFLAGS" directive. The Apple linker recombines the separated arguments "-l" and "to_library" into "-lto_library", which loads an arbitrary LTO plugin dynamic library at link time. The go command now disallows "-l" followed by "to_library" in "#cgo LDFLAGS", rejects "-lazy_library", "-lazy_framework", "-load_hidden", "-library", and "-library-path", and checks recombined "-l" arguments against disallowed linker flags and patterns.