Arm backend: Fix pre-push copyright header detection

The license check used any Arm substring in the first header lines to
decide whether a file had an Arm copyright header.

That misclassified files with includes such as arm_neon.h as having
an Arm header, then failed them when the current Arm year was absent.

Match the actual Arm copyright line instead so non-Arm headers are
skipped while real Arm headers still get the year check.

Signed-off-by: Per Held <per.held@arm.com>
Change-Id: Iafda07d8e2cf379672939a268fc3c39fc0ab895e

Author: perheld

backends/arm/scripts/pre-push

@@ -184,7 +184,8 @@ for COMMIT in ${COMMITS}; do
         esac
 
         file_header=$(head "$committed_file")
-        if ! echo "$file_header" | grep -qi "Arm"; then
+        arm_copyright_regex="Copyright .*Arm Limited and/or its affiliates"
+        if ! echo "$file_header" | grep -Eqi "$arm_copyright_regex"; then
             echo -e "${WARNING} No Arm copyright header in ${committed_file}"\
             " (skipping license year check)"
             continue