ART: String DexCache asm fast path for x86

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: Id0c337b380adc0b2bde5325745c928290d9da1b0