GO-2026-6850: 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:cgoimportdynamic", "//go:cgoexportdynamic", "//go:cgoexportstatic", "//go:cgoimportstatic", and "//go:cgodynamiclinker" directives, and the linker passed the dynamic linker path using "-Wl,--dynamic-linker,<path>". Furthermore, the two-argument form of "//go:cgoimportdynamic" (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 "cgoimportdynamic" no longer overwrites symbols exported via "cgoexportdynamic" or "cgoexportstatic".