Fix link flags in Bazel build

Critical flags were not being applied to link steps. This applies -mcpu
and -mthumb to the link steps to make the produced binaries work again.

bazel/toolchain/BUILD.bazel

@@ -5,7 +5,10 @@ load("@rules_cc//cc/toolchains:toolchain.bzl", "cc_toolchain")
 
 cc_args(
     name = "cortex-m0",
-    actions = ["@rules_cc//cc/toolchains/actions:compile_actions"],
+    actions = [
+        "@rules_cc//cc/toolchains/actions:compile_actions",
+        "@rules_cc//cc/toolchains/actions:link_actions",
+    ],
     args = [
         "-mcpu=cortex-m0plus",
         "-mthumb",