Implement external linking for mips64. This involves completing the functions needed for external linking in src/cmd/link/internal/mips64/asm.go, including elfreloc1 and archreloc.

(My original comment included cgo and internal linking. I have revised it to cover only external linking, and opened #14126 for complete cgo with external linking.)

I think that's a little inaccurate and muddles implementing cgo and external linking. External linking mostly just requires elfreloc1 (it might also require changes to the entrypoint?). elfsetupplt, adddynsym, adddynrela are required for cgo with internal linking (they are not implemented for arm64, for example, which only supports cgo via external linking).

The runtime stuff in asm_mips64x.s is required for either flavour of cgo, but not external linking.

Thanks @mwhudson for helping un-muddle my understanding of cgo vs. internal linking vs. external linking. So to build a Go program with embedded C code (via extern "C") that calls functions in an external library, I need just

• cgo runtime support in asm_mips64x.s
• external linking support in elfreloc1

but not the internal linking support in elfsetupplt, adddynsym, adddynrela. Is that correct?

I reverted the issue description to cover only external linking, and opened #14126 for complete cgo support.

CL https://golang.org/cl/19803 mentions this issue.