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`.

test/standalone/stack_iterator/build.zig

@@ -22,7 +22,7 @@ pub fn build(b: *std.Build) void {
             .root_source_file = .{ .path = "unwind.zig" },
             .target = target,
             .optimize = optimize,
-            .unwind_tables = target.target.isDarwin(),
+            .unwind_tables = target.result.isDarwin(),
             .omit_frame_pointer = false,
         });
 
@@ -70,7 +70,7 @@ pub fn build(b: *std.Build) void {
             .strip = false,
         });
 
-        if (target.target.os.tag == .windows)
+        if (target.result.os.tag == .windows)
             c_shared_lib.defineCMacro("LIB_API", "__declspec(dllexport)");
 
         c_shared_lib.addCSourceFile(.{
@@ -84,7 +84,7 @@ pub fn build(b: *std.Build) void {
             .root_source_file = .{ .path = "shared_lib_unwind.zig" },
             .target = target,
             .optimize = optimize,
-            .unwind_tables = target.target.isDarwin(),
+            .unwind_tables = target.result.isDarwin(),
             .omit_frame_pointer = true,
         });