Allow compiling and linking to be done on separate platforms (#53)

Because of OS cross compile limitations in mojo (see previous commits),
compile actions must be pinned to the target OS. Because the mojo
toolchain was set on the top level rule, all resolved toolchains were
pinned to the target OS, which meant linking could not be cross
compiled. This removes that restriction using exec_groups to free up
macOS CPU time when cross compiling.

Author: keith

mojo/private/mojo_binary_test.bzl

@@ -44,10 +44,13 @@ then be used in copts with the $(location) function.
 }
 
 _TOOLCHAINS = use_cpp_toolchain() + [
-    "//:toolchain_type",
     _PYTHON_TOOLCHAIN_TYPE,
 ]
 
+_EXEC_GROUPS = {
+    "compile": exec_group(toolchains = ["//:toolchain_type"]),
+}
+
 def _find_main(name, srcs, main):
     """Finds the main source file from the list of srcs and the main attribute."""
     if main:
@@ -75,7 +78,7 @@ def _find_main(name, srcs, main):
 
 def _mojo_binary_test_implementation(ctx, *, shared_library = False):
     cc_toolchain = find_cpp_toolchain(ctx)
-    mojo_toolchain = ctx.toolchains["//:toolchain_type"].mojo_toolchain_info
+    mojo_toolchain = ctx.exec_groups["compile"].toolchains["//:toolchain_type"].mojo_toolchain_info
     py_toolchain = ctx.toolchains[_PYTHON_TOOLCHAIN_TYPE]
 
     object_file = ctx.actions.declare_file(ctx.label.name + ".lo")
@@ -130,6 +133,7 @@ def _mojo_binary_test_implementation(ctx, *, shared_library = False):
             "TEST_TMPDIR": ".",
         },
         use_default_shell_env = True,
+        exec_group = "compile",
         toolchain = "//:toolchain_type",
     )
 
@@ -266,6 +270,7 @@ def _mojo_binary_test_implementation(ctx, *, shared_library = False):
 mojo_binary = rule(
     implementation = lambda ctx: _mojo_binary_test_implementation(ctx),
     attrs = _ATTRS,
+    exec_groups = _EXEC_GROUPS,
     toolchains = _TOOLCHAINS,
     fragments = ["cpp"],
     executable = True,
@@ -274,6 +279,7 @@ mojo_binary = rule(
 mojo_test = rule(
     implementation = lambda ctx: _mojo_binary_test_implementation(ctx),
     attrs = _ATTRS,
+    exec_groups = _EXEC_GROUPS,
     toolchains = _TOOLCHAINS,
     fragments = ["cpp"],
     test = True,
@@ -287,6 +293,7 @@ mojo_shared_library = rule(
             doc = "The name of the shared library to be created.",
         ),
     },
+    exec_groups = _EXEC_GROUPS,
     toolchains = _TOOLCHAINS,
     fragments = ["cpp"],
 )