Host linker flag injection via //go:cgo_* directives in cmd/compile and cmd/link

Building a malicious package containing "//go:cgo_*" compiler directives could inject arbitrary flags into the external linker invocation on ELF platforms. The compiler and linker did not reject commas in "//go:cgo_import_dynamic", "//go:cgo_export_dynamic", "//go:cgo_export_static", "//go:cgo_import_static", and "//go:cgo_dynamic_linker" directives, and the linker passed the dynamic linker path using "-Wl,--dynamic-linker,<path>". Furthermore, the two-argument form of "//go:cgo_import_dynamic" (which is permitted in non-cgo Go source files) could overwrite the external symbol name of a symbol already marked for cgo export. A symbol name or dynamic linker path containing a comma or leading "@" could split "-Wl,--export-dynamic-symbol=" or "-Wl,--dynamic-linker," into arbitrary host linker flags or response file arguments. The compiler and linker now reject commas in "//go:cgo_*" symbol and dynamic linker directives, the linker passes "-Wl,--dynamic-linker=<path>", and "cgo_import_dynamic" no longer overwrites symbols exported via "cgo_export_dynamic" or "cgo_export_static".