ART: refactor x86/x86-64 entrypoints

This patch:

- removes unused stubs in x86/64 runtimes (art_quick_l2d, art_quick_l2f
  and art_quick_idivmod)
- replaces art_quick_fmod, art_quick_fmodf and art_quick_is_assignable
  entrypoints in x86-64 to the direct calls
- removes art_quick_indexof stub in x86-64

Change-Id: I6141c5c73b0b449fa3b866068b101e0be211b93e
Signed-off-by: Alexei Zavjalov <alexei.zavjalov@intel.com>