[NFC] [test] [libcxx] Fix invalid escape sequences (#168636)

```
>>> "_target-has-llvm-21 || target={{.+}}-apple-macosx{{26.[0-9](.\d+)?}}" == r"_target-has-llvm-21 || target={{.+}}-apple-macosx{{26.[0-9](.\\
d+)?}}"
<python-input-6>:1: SyntaxWarning: invalid escape sequence '\d'
True
>>> "_target-has-llvm-20 || target={{.+}}-apple-macosx{{15.[4-9](.\d+)?}}" == r"_target-has-llvm-20 || target={{.+}}-apple-macosx{{15.[4-9](.\\
d+)?}}"
<python-input-7>:1: SyntaxWarning: invalid escape sequence '\d'
True
>>> "_target-has-llvm-19 || target={{.+}}-apple-macosx{{15.[0-3](.\d+)?}}" == r"_target-has-llvm-19 || target={{.+}}-apple-macosx{{15.[0-3](.\\
d+)?}}"
<python-input-8>:1: SyntaxWarning: invalid escape sequence '\d'
True
>>> "_target-has-llvm-18 || target={{.+}}-apple-macosx{{14.[4-9](.\d+)?}}" == r"_target-has-llvm-18 || target={{.+}}-apple-macosx{{14.[4-9](.\\
d+)?}}"
<python-input-9>:1: SyntaxWarning: invalid escape sequence '\d'
True
```
Signed-off-by: Hafidz Muzakky <[email protected]>

Author: fmayer

libcxx/utils/libcxx/test/features/availability.py

@@ -29,28 +29,28 @@
     Feature(
         name="_target-has-llvm-20",
         when=lambda cfg: BooleanExpression.evaluate(
-            "_target-has-llvm-21 || target={{.+}}-apple-macosx{{26.[0-9](.\d+)?}}",
+            r"_target-has-llvm-21 || target={{.+}}-apple-macosx{{26.[0-9](.\d+)?}}",
             cfg.available_features,
         ),
     ),
     Feature(
         name="_target-has-llvm-19",
         when=lambda cfg: BooleanExpression.evaluate(
-            "_target-has-llvm-20 || target={{.+}}-apple-macosx{{15.[4-9](.\d+)?}}",
+            r"_target-has-llvm-20 || target={{.+}}-apple-macosx{{15.[4-9](.\d+)?}}",
             cfg.available_features,
         ),
     ),
     Feature(
         name="_target-has-llvm-18",
         when=lambda cfg: BooleanExpression.evaluate(
-            "_target-has-llvm-19 || target={{.+}}-apple-macosx{{15.[0-3](.\d+)?}}",
+            r"_target-has-llvm-19 || target={{.+}}-apple-macosx{{15.[0-3](.\d+)?}}",
             cfg.available_features,
         ),
     ),
     Feature(
         name="_target-has-llvm-17",
         when=lambda cfg: BooleanExpression.evaluate(
-            "_target-has-llvm-18 || target={{.+}}-apple-macosx{{14.[4-9](.\d+)?}}",
+            r"_target-has-llvm-18 || target={{.+}}-apple-macosx{{14.[4-9](.\d+)?}}",
             cfg.available_features,
         ),
     ),