improve detection of how to execute binaries on the host

`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.

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