Linker flag injection via cgo_import_dynamic and PE DLL names in cmd/cgo, cmd/compile, and cmd/link

Building a malicious package that uses "//go:cgo_import_dynamic" or links against a crafted PE object on Windows could pass unsafe library arguments to the external linker. The compiler only checked the first character of the library name in "//go:cgo_import_dynamic" directives and skipped validation in cgo-generated files. In addition, on Windows, "cmd/cgo" and "cmd/link" did not validate imported DLL names extracted from PE object files before emitting "-l<name>" flags for the external linker, allowing a crafted DLL name containing ".." path traversal or special characters to reference arbitrary files on disk. The compiler now validates every character of the library name in "//go:cgo_import_dynamic" directives (including in cgo-generated files on Windows) and rejects ".." path traversal, and "cmd/cgo" and "cmd/link" reject PE imported DLL names containing path separators, "..", ":", or leading "-" or "@".