cmd/ld: invoke host linker for Windows cgo build #4069
Description
It should be possible to invoke the host linker for a cgo build. That is, we'd run 6l to write a .o file for the Go pieces and then invoke gcc to do the final link with the .syso files. This would fix many corner cases in 6l's support for linking arbitrary ELF objects into the binary. I would like to keep 6l's arbitrary ELF support so that one need not have a C compiler installed to use a Go compiler, but we need the 'invoke gcc' plan as a fallback. 6l = 6l, 5l, 8l here, to avoid confusion with host ld.