riscv64 support for gitlab-runners

Release notes

{placeholder for release notes}

Description

We'd like to use GitLab Runner on RISC-V hardware for some internal projects. Sadly, it does not build for linux/riscv64 currently.

I have tried adding linux/riscv64 to what appear to be the appropriate places in the Makefile, and patching gox to accept linux/riscv64 as -osarch (it simply returns success having done nothing if you pass it something not in its blessed list). gitlab-runner continues to build for current architectures, but the RISC-V version fails:

1 errors occurred:
--> linux/riscv64 error: exit status 2
Stderr: # github.com/kr/pty
../../../go/pkg/mod/github.com/kr/pty@v1.1.1/pty_linux.go:39:8: undefined: _C_uint
../../../go/pkg/mod/github.com/kr/pty@v1.1.1/pty_linux.go:48:8: undefined: _C_int

Sadly at this point my knowledge of Go was exhausted.

Proposal

Assuming the problem is as simple as I am ignorant, adjust the gitlab-runner build tooling to allow building for RISC-V.

Links to related issues and merge requests / references