Use strcmp() for checking @{Fast,Critical}Native annotations.

Instead of looking up the descriptor in boot class path
loader (where @{Fast,Critical}Native are guaranteed to
be already resolved) and then checking if it's the
@{Fast,Critical}Native annotation, just check the
descriptor with strcmp().

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 65574695
Change-Id: I765590d039981d169fb3c606b6166580a84303b6