Switch to a new cortex-m4 compiler

This one is newer, so it supports a newer C++ standard. It's also
hermetic.

Also add a variant for building for some Kinetis-K chips with less RAM
that are used on fet12v2.

Change-Id: I9e50b6aae498e0c35acfedb846b3ada619a0e630
diff --git a/tools/cpp/static_crosstool.pb b/tools/cpp/static_crosstool.pb
index 99a36f9..5cfe31c 100644
--- a/tools/cpp/static_crosstool.pb
+++ b/tools/cpp/static_crosstool.pb
@@ -27,6 +27,11 @@
   toolchain_identifier: "cortex-m4f"
 }
 
+default_toolchain {
+  cpu: "cortex-m4f-k22"
+  toolchain_identifier: "cortex-m4f-k22"
+}
+
 toolchain {
   abi_version: "armeabi-v7a"
   abi_libc_version: "armeabi-v7a"