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/BUILD.gn b/BUILD.gn
index cb7f78c..d5d3032 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -263,7 +263,7 @@
       "$dir_pw_checksum:tests",
       "$dir_pw_chrono:tests",
       "$dir_pw_containers:tests",
-      "$dir_pw_cpu_exception_armv7m:tests",
+      "$dir_pw_cpu_exception_cortex_m:tests",
       "$dir_pw_fuzzer:tests",
       "$dir_pw_hdlc:tests",
       "$dir_pw_hex_dump:tests",