internal/buildcfg: add ability to get GORISCV64 variable in GOGOARCH
For #61476
Change-Id: I29f4c1c3c3303e70ec2d7f380112eb2d00754018
Reviewed-on: https://go-review.googlesource.com/c/go/+/665655 Reviewed-by: Mark Ryan <markdryan@rivosinc.com> Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Mark Freeman <mark@golang.org> Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com>