[ARM] Make testcase warning pattern match more specific

Make the warning more specific as downstream compilers could produce other warnings.

Reviewed By: tstellar

Differential Revision: https://reviews.llvm.org/D122487

Author: rs-arm

clang/test/Driver/aarch64-security-options.c

@@ -52,4 +52,4 @@
 
 // Check that the linker driver doesn't warn about -mbranch-protection=standard
 // as an unused option.
-// LINKER-DRIVER-NOT: warning:
+// LINKER-DRIVER-NOT: warning: argument unused during compilation: '-mbranch-protection=standard'