Fix Python SyntaxWarning in two f-string literals.

Building on Ubuntu 24.04, multilib-generate.py provokes some
SyntaxWarning messages from Python 3.12, because of cases where we'd
written `\+` in a string, intended to be emitted literally. 3.12 still
_does_ emit it literally, but prints a warning pointing out that we
should properly escape the backslash, writing it as `\\+`.

Author: statham-arm

multilib-generate.py

@@ -247,10 +247,10 @@ def generate_extensions(args):
     print("# original option to check that.")
 
     for feature in all_features:
-        print(f"- Match: -march=armv.*\\+{feature}($|\+.*)")
+        print(f"- Match: -march=armv.*\\+{feature}($|\\+.*)")
         print(f"  Flags:")
         print(f"  - -march=armvX+{feature}")
-        print(f"- Match: -march=armv.*\\+no{feature}($|\+.*)")
+        print(f"- Match: -march=armv.*\\+no{feature}($|\\+.*)")
         print(f"  Flags:")
         print(f"  - -march=armvX+no{feature}")
     print()