Make sure the ParsedRegex structure has the right size.

Some versions of GCC/g++ will not necessarily pad the structure to
a 32-bit boundary, so make the alignment and the filler explicit.

Detected on: Debian/m68k; https://buildd.debian.org/status/fetch.php?pkg=kakoune&arch=m68k&ver=2020.09.01-1&stamp=1629387444&raw=0

src/regex_impl.cc

@@ -81,7 +81,8 @@ struct ParsedRegex
         NodeIndex children_end;
         Codepoint value;
         Quantifier quantifier;
-    };
+        uint16_t filler = 0;
+    } __attribute__((packed));
     static_assert(sizeof(Node) == 16, "");
 
     Vector<Node, MemoryDomain::Regex> nodes;