ART: String DexCache asm fast path for x86_64

In the entrypoints, before a string is looked up in the slow
path (in the intern table), I added assembly to check the dex cache and return a
string pointer if the string is already in the dex cache.

Test: test-art-host

Change-Id: Id1b0314d597c2abff3f7d332f4ab66271f026dc1