Move the cros_* GN args to a .gni file and add a bootstrap cros toolchain.

build/config/compiler/compiler.gni

Index: build/config/compiler/compiler.gni
diff --git a/build/config/compiler/compiler.gni b/build/config/compiler/compiler.gni
index 845bcdec531d7a99981fbd2bcb303d56b69c45fd..ca677f7a3837ad2a7fc8e85d3a6133a01f026ae2 100644
--- a/build/config/compiler/compiler.gni
+++ b/build/config/compiler/compiler.gni
@@ -37,6 +37,18 @@ declare_args() {
   } else {
     full_wpo_on_official = false
   }
+
+  # use_debug_fission: whether to use split DWARF debug info
+  # files. This can reduce link time significantly, but is incompatible
+  # with some utilities such as icecc and ccache. Requires gold and
+  # gcc >= 4.8 or clang.
+  # http://gcc.gnu.org/wiki/DebugFission
+  #
+  # This is a placeholder value indicating that the code below should set
+  # the default.  This is necessary to delay the evaluation of the default
+  # value expression until after its input values such as use_gold have
+  # been set, e.g. by a toolchain_args() block.
+  use_debug_fission = "default"
 }
 
 declare_args() {