pw_cpu_exception_cortex_m: Move armv7m->cortex_m

Moves the armv7m implementation of pw_cpu_exception to
pw_cpu_exception_cortex_m since much of the logic can be shared between
ARMv8-M and ARMv7-M. Symbol names are also updated to reflect current
naming style for extern "C" symbols. Old names will be removed in
pwrev/31060.

Warning, symbol renames are breaking!
pw_CpuExceptionEntry -> pw_cpu_exception_Entry
pw_CpuExceptionDefaultHandler -> pw_cpu_exception_DefaultHandler

Change-Id: I7ab99ac7637c436ba959d7546043090c66fe4215
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/30922
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Ewout van Bekkum <ewout@google.com>
Pigweed-Auto-Submit: Armando Montanez <amontanez@google.com>
diff --git a/pw_cpu_exception_cortex_m/CMakeLists.txt b/pw_cpu_exception_cortex_m/CMakeLists.txt
new file mode 100644
index 0000000..c434a21
--- /dev/null
+++ b/pw_cpu_exception_cortex_m/CMakeLists.txt
@@ -0,0 +1,20 @@
+# Copyright 2020 The Pigweed Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may not
+# use this file except in compliance with the License. You may obtain a copy of
+# the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations under
+# the License.
+
+include($ENV{PW_ROOT}/pw_build/pigweed.cmake)
+
+pw_auto_add_simple_module(pw_cpu_exception_cortex_m
+  IMPLEMENTS_FACADE
+    pw_cpu_exception
+)