cmd/cgo: cgo with external linking for linux/mips64

[eswierk]

Implement cgo with external linking for linux/mips64 and linux/mips64le (MIPS N64 ELF ABI, big- and little-endian). Work includes:

• complete external linking support (cmd/link: external link for mips64 #12560)
• fill in the portions of src/runtime/asm_mips64x.s marked "no cgo yet", plus whatever else cgo requires
• enable cgo external link tests, and get (cd misc/cgo/test && go test -linkmode=external) to pass on Linux on mips64 and mips64le hardware

Initial testing can be done using qemu-user for mips64 and mips64le targets running on any host. Skyport Systems (my employer) can also provide access to mips64le hardware for testing.

See #8912 for a description of similar work for ppc64 (though internal linking support is optional/extra credit). For a working example, browse the current arm64 code, which supports exactly the functionality sought for mips64 (cgo with external linking, not internal linking).

[eswierk]

Background info, for the benefit of a potential bounty hunter...

How cgo works, from the horse’s mouth. Plus an explanation of golang's odd x86-ish assembly language. Some of the links to Plan9 docs are broken; use this archive instead.

The Altoros blog series on golang internals has lots of useful info about code generation, linking, relocations, assembly.

Explanation of golang’s clever conditional compilation features (build tags and magic filenames).

For brushing up on MIPS trivia: N32/N64 ABI and other ABIs, ELF relocation types.

There is probably some overlap with the voodoo that needs to be added to libffi to support closures with gccgo on mips64.

[minux]

The cgo support is not very difficult, but would it be possible for you contribute a big endian linux/mips64 builder? We need one to move forward with the port. Thanks a lot.

[eswierk]

@minux We only have little endian mips64 currently. Would that be useful to you?

[gopherbot]

CL https://golang.org/cl/19809 mentions this issue.