configure: filter out -guard:signret from armasm flags

While cl.exe supports -guard:signret, armasm64 complains about
unknown flag. Note that -guard:ehcont is accepted by armasm64.

Fixes:
error A2029: unknown command-line argument or argument value -guard:signret

Signed-off-by: Kacper Michajłow <kasper93@gmail.com>

configure

@@ -4968,6 +4968,7 @@ armasm_flags(){
             # Filter out MSVC cl.exe options from cflags that shouldn't
             # be passed to gas-preprocessor
             -M[TD]*)                                            ;;
+            -guard:signret)                                     ;;
             *)                  echo $flag                      ;;
         esac
    done