Keep track of RelocInfo::Mode for ConstantPoolEntries in the assembler,
so that ARM's constant pool de-duping does not accidentally dedupe
constants with the same value but different reloc modes (e.g. the first
Code object in the builtins table as a CODE_TARGET vs. the builtin table
itself as an EXTERNAL_REFERENCE).

Change-Id: I15fad5b83bb99688726e66e0e290149025c6c059
Reviewed-on: https://chromium-review.googlesource.com/958864Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51900}