[scudo] Make comment compatible with gcc (#106137)

gcc interprets a backslash '\\' as the last char before a new line as a
line continuation character, even in a comment context. This can produce
an "error: multi-line comment [-Werror=comment]".

This PR adds delimiters so that the comment can compile with gcc.

Author: Caslyn

compiler-rt/lib/scudo/standalone/secondary.h

@@ -426,7 +426,7 @@ class MapAllocatorCache {
     //  The difference between the retrieved memory chunk and the request
     //  size is at most MaxAllowedFragmentedPages
     //
-    //  / MaxAllowedFragmentedPages * PageSize \
+    // +- MaxAllowedFragmentedPages * PageSize -+
     // +--------------------------+-------------+
     // |                          |             |
     // +--------------------------+-------------+