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/pw_log_basic/BUILD.gn b/pw_log_basic/BUILD.gn
index e04bd5c..cfeb7df 100644
--- a/pw_log_basic/BUILD.gn
+++ b/pw_log_basic/BUILD.gn
@@ -26,15 +26,13 @@
   include_dirs = [ "public_overrides" ]
 }
 
-if (dir_pw_log_backend == dir_pw_log_basic) {
-  pw_source_set("pw_log_basic") {
-    public_configs = [
-      ":backend_config",
-      ":default_config",
-    ]
-    public = [ "public_overrides/pw_log_backend/log_backend.h" ]
-    public_deps = [ ":core" ]
-  }
+pw_source_set("pw_log_basic") {
+  public_configs = [
+    ":backend_config",
+    ":default_config",
+  ]
+  public = [ "public_overrides/pw_log_backend/log_backend.h" ]
+  public_deps = [ ":core" ]
 }
 
 pw_source_set("core") {