Fix building with -c dbg

Just create separate lists of flags (and corresponding features) for
each compilation mode, instead of trying to be cute with a default that
gets overriden.

Change-Id: Ib9741681030f5b88dcf200f0144156a626bc16aa
Signed-off-by: Brian Silverman <bsilver16384@gmail.com>
diff --git a/third_party/bazel-toolchain/toolchain/rules.bzl b/third_party/bazel-toolchain/toolchain/rules.bzl
index f27e387..aad2d37 100644
--- a/third_party/bazel-toolchain/toolchain/rules.bzl
+++ b/third_party/bazel-toolchain/toolchain/rules.bzl
@@ -175,6 +175,13 @@
                "({}), ".format(", ".join(_supported_os_arch_keys())) +
                "used only with -c dbg."),
     ),
+    "fastbuild_copts": attr.string_list_dict(
+        mandatory = False,
+        doc = ("Extra flags for compiling C, C++, and assembly files, " +
+               "for each target OS and arch pair you want to support " +
+               "({}), ".format(", ".join(_supported_os_arch_keys())) +
+               "used only with -c fastbuild."),
+    ),
     "linkopts": attr.string_list_dict(
         mandatory = False,
         doc = ("Extra flags to pass to the linker, " +