cmd/dist: baked-in settings need to be per-GOOS/GOARCH

[rsc]

The settings CGO_ENABLED, GO_LDSO, and GO_EXTLINK_ENABLED are baked into the toolchain built by cmd/dist without reference to which GOOS/GOARCH they are appropriate for. This results in toolchains that may use inappropriate settings for cross-compiling. The same is true of CC and CXX, but there we also have CC_FOR_TARGET, CC_FOR_${GOOS}_${GOARCH}, and so on. We should probably do the same for these other settings. Doing it for CGO_ENABLED would allow building a cross-compiled toolchain that uses cgo on the target system, which is necessary (and perhaps sufficient) for #53862.

I will take a look at this. I filed a separate bug #54197 for not baking in GO_LDSO at all.

[rsc]

CGO_ENABLED turns out to be OK because you can run make.bash with CGO_ENABLED="" to get "automatic" mode.
The changes I made for #54197 end up doing the same for GO_LDSO.
That leaves GO_EXTLINK_ENABLED, which is rarely set explicitly either.
So there may not be anything to do here after all.