Define Linux clang target as toolchain

This change configures the Linux clang -Og target as a toolchain with
args. To support this, all Pigweed variables from pw_vars_default are
made into build args, defined within their respective modules.

The Linux clang target/toolchain is the only one that currently works.
Pigweed's other targets will be ported in later changes.

Change-Id: I051e29fde7577e41a3184c37031a4e04936404e1
diff --git a/targets/host/linux/linux.gni b/targets/host/linux/linux.gni
new file mode 100644
index 0000000..c81bb37
--- /dev/null
+++ b/targets/host/linux/linux.gni
@@ -0,0 +1,18 @@
+# Copyright 2020 The Pigweed Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may not
+# use this file except in compliance with the License. You may obtain a copy of
+# the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations under
+# the License.
+
+import("../host_common.gni")
+
+pw_bloat_BLOATY_CONFIG = get_path_info("linux.bloaty", "abspath")
+pw_unit_test_AUTOMATIC_RUNNER = get_path_info("../run_test", "abspath")