pw_build, pw_toolchain: Add default configs

This CL reworks toolchains to be defined as scopes composing configs and
toolchain args. This allows individual targets to add configs to or
remove configs from the defaults.

Change-Id: I105a231f3e2de35499e1617241f3eeb42ef3248d
diff --git a/pw_assert/BUILD.gn b/pw_assert/BUILD.gn
index 63b61b6..ea05d38 100644
--- a/pw_assert/BUILD.gn
+++ b/pw_assert/BUILD.gn
@@ -49,6 +49,9 @@
     "pw_assert_test/fake_backend.h",
   ]
   deps = [ dir_pw_status ]
+
+  # TODO(frolv): Fix this test on the QEMU target.
+  enable_if = pw_executable_config.target_type != "lm3s6965evb_executable"
 }
 
 pw_test("assert_backend_compile_test") {