Add missing hint markers in C feature files

Author: cdervis

features_c23.yaml

@@ -224,7 +224,7 @@ features:
     paper: N2900
     content: c-empty-init.md
     support:
-      - GCC 13
+      - GCC 13 (hint)
       - Clang 17 (hint)
       - Xcode (partial)
     hints:

features_c2y.yaml

@@ -52,9 +52,9 @@ features:
   - desc: "Remove imaginary types"
     paper: N3274
     support:
-      - GCC
-      - Clang
-      - Xcode
+      - GCC (hint)
+      - Clang (hint)
+      - Xcode (hint)
     hints:
       - target: GCC
         msg: "GCC has never supported imaginary types."