std.Build.ResolvedTarget: rename target field to result

This change is seemingly insignificant but I actually agonized over this
for three days. Some other things I considered:

* (status quo in master branch) make Compile step creation functions
  accept a Target.Query and delete the ResolvedTarget struct.
  - downside: redundantly resolve target queries many times
* same as before but additionally add a hash map to cache target query
  resolutions.
  - downside: now there is a hash map that doesn't actually need to
    exist, just to make the API more ergonomic.
* add is_native_os and is_native_abi fields to std.Target and use it
  directly as the result of resolving a target query.
  - downside: they really don't belong there. They would be available
    as comptime booleans via `@import("builtin")` but they should not
    be exposed that way.

With this change the downsides are:
* the option name of addExecutable and friends is `target` instead of
  `resolved_target` matching the type name.
  - upside: this does not break compatibility with existing build
    scripts
* you likely end up seeing `target.result.cpu.arch` rather than
  `target.cpu.arch`.
  - upside: this is an improvement over `target.target.cpu.arch` which
    it was before this commit.
  - downside: `b.host.target` is now `b.host.result`.

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