Log Message:
lang/go: Add cross-build support.

This adds cross-build support for lang/go123 and for the Go-related
infrastructure in pkgsrc.  (We could do older versions of Go too with
a little more work.)

Noteworthy changes that are not conditional on USE_CROSS_COMPILE:

1. lang/go/version.mk is rearranged to be more data-driven than
   conditional-driven.  Making it data-driven makes it easier to
   define both GOARCH and GOHOSTARCH from the same tables when
   MACHINE_ARCH and NATIVE_MACHINE_ARCH are not the same.

   This is a likely source of broken edge cases.  I went through the
   old conditional logic and hand-checked all the conditions but I
   could have made a mistake.

2. go-module.mk and go-package.mk define GOPATH_BIN to be `bin' for
   native builds, and `bin/${GO_PLATFORM}' for cross builds -- this
   is the subdirectory of GOPATH where the Go toolchain puts binaries
   so that packages with custom do-install targets can avoid any need
   for USE_CROSS_COMPILE conditionals.

   The default do-install targets use pax slightly differently now to
   avoid the need for USE_CROSS_COMPILE conditionals.  I think the
   logic is equivalent for native builds but this is worth reviewing.

3. lang/go123 no longer depends on bash and Perl at runtime.  As far
   as I can tell, this was just a kludge to pacify check-interpreter
   complaints in the copy of the source code that Go ships under
   ${PREFIX}/go123/src.  We don't need to replace the interpreter at
   build-time -- most of these scripts are not run at all during the
   build, and the handful that remain (make.bash, run.bash) are run
   with ${BASH}.  Instead, we CHECK_INTERPRETER_SKIP them in the
   installed copy of the source code.

Proposed on tech-pkg:
https://mail-index.netbsd.org/tech-pkg/2025/01/19/msg030395.html