`getExternalExecutor` is moved from `std.zig.CrossTarget` to
`std.zig.system.NativeTargetInfo.getExternalExecutor`.
The function also now communicates a bit more information about *why*
the host is unable to execute a binary. The CLI is updated to report
this information in a useful manner.
`getExternalExecutor` is also improved to detect such patterns as:
* x86_64 is able to execute x86 binaries
* aarch64 is able to execute arm binaries
* etc.
Added qemu-hexagon support to `getExternalExecutor`.
`std.Target.canExecBinaries` of is removed; callers should use the more
powerful `getExternalExecutor` instead.
Now that `zig test` tries to run the resulting binary no matter what,
this commit has a follow-up change to the build system and docgen to
utilize the `getExternalExecutor` function and pass `--test-no-exec`
in some cases to avoid getting the error.
Additionally:
* refactor: extract NativePaths and NativeTargetInfo into their own
files named after the structs.
* small improvement to langref to reduce the complexity of the `callconv`
expression in a couple examples.
