instrumented_libraries: Add padding to RPATHs to allow patching later.

R=eugenis@chromium.org

Change-Id: Iff7213d5d2b483fe8c51cbe20ac15952719fdccb
Reviewed-on: https://chromium-review.googlesource.com/936881Reviewed-by: Evgeniy Stepanov <eugenis@chromium.org>
Commit-Queue: Oliver Chang <ochang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539332}

third_party/instrumented_libraries/BUILD.gn

@@ -85,6 +85,8 @@ if (prebuilt_instrumented_libraries_available) {
       # DSOs before system-installed versions in library search path.
       "-Wl,-R,\$ORIGIN/instrumented_libraries_prebuilt/$sanitizer_type/lib",
       "-Wl,-z,origin",
+      # Add some padding to allow RPATHs to be modified later.
+      "-Wl,-R,________________________________________________________________PADDING________________________________________________________________"
     ]
   }
 }
@@ -173,6 +175,8 @@ if (use_locally_built_instrumented_libraries) {
       # DSOs before system-installed versions in library search path.
       "-Wl,-R,\$ORIGIN/instrumented_libraries/${sanitizer_type}/lib",
       "-Wl,-z,origin",
+      # Add some padding to allow RPATHs to be modified later.
+      "-Wl,-R,________________________________________________________________PADDING________________________________________________________________"
     ]
   }