cmd/link/internal/ld: emit better complex types for COFF symbols
The Go linker has always used IMAGE_SYM_TYPE_NULL as COFF symbol
type [1] when external linking and array of structs
(IMAGE_SYM_DTYPE_ARRAY<<4+IMAGE_SYM_TYPE_STRUCT) when internal linking.
This behavior seems idiosyncratic, and looking at the git history it
seems that it has probably been cargo culted from earlier toolchains.
This CL updates the Go linker to use IMAGE_SYM_DTYPE_FUNCTION<<4 for
those symbols representing functions, and IMAGE_SYM_TYPE_NULL otherwise.
This new behavior better represents the symbol types, and can help
other tools interpreting the intent of each symbol, e.g. debuggers or
tools extracting debug info from Go binaries. It also mimics what other
toolchains do, i.e. MSVC, LLVM, and GCC.