go: enable MIPS GOARCH/cross integration

This allows the Go compiler in nixpkgs (eg. buildGoModule) to work with
crossSystem.config == mips-*, eg mips-unknown-linux-musl, and
succesfully generate Go MIPS binaries.

nix-build -A grpcurl --arg crossSystem '{ config = "mips-unknown-linux-musl"; }'

This unfortunately cannot currently be tested on qemu-mips as Go emits
ELF files that fail to execute correctly in qemu-user (see:
https://go-review.googlesource.com/c/go/+/239217, on track to land in Go
1.17). However, I have tested this on a physical MIPS device.

I have not been able to build anything using cgo (hit various
compilation errors in C dependencies), but considering
mips-unknown-linux-musl is not a support nixpkgs target this isn't that
surprising.

pkgs/development/compilers/go/1.14.nix

@@ -41,6 +41,7 @@ let
     "armv6l" = "arm";
     "armv7l" = "arm";
     "powerpc64le" = "ppc64le";
+    "mips" = "mips";
   }.${platform.parsed.cpu.name} or (throw "Unsupported system");
 
   # We need a target compiler which is still runnable at build time,

pkgs/development/compilers/go/1.15.nix

@@ -41,6 +41,7 @@ let
     "armv6l" = "arm";
     "armv7l" = "arm";
     "powerpc64le" = "ppc64le";
+    "mips" = "mips";
   }.${platform.parsed.cpu.name} or (throw "Unsupported system");
 
   # We need a target compiler which is still runnable at build time,

pkgs/development/compilers/go/1.16.nix

@@ -41,6 +41,7 @@ let
     "armv6l" = "arm";
     "armv7l" = "arm";
     "powerpc64le" = "ppc64le";
+    "mips" = "mips";
   }.${platform.parsed.cpu.name} or (throw "Unsupported system");
 
   # We need a target compiler which is still runnable at build time,