Change 64 bit ArtMethod fields to be pointer sized

Change 64 bit ArtMethod fields to be pointer sized

Changed the 64 bit entrypoint and gc map fields in ArtMethod to be
pointer sized. This saves a large amount of memory on 32 bit systems.
Reduces ArtMethod size by 16 bytes on 32 bit.

Total number of ArtMethod on low memory mako: 169957
Image size: 49203 methods -> 787248 image size reduction.
Zygote space size: 1070 methods -> 17120 size reduction.
App methods: ~120k -> 2 MB savings.

Savings per app on low memory mako: 125K+ per app
(less active apps -> more image methods per app).

Savings depend on how often the shared methods are on dirty pages vs
shared.

TODO in another CL, delete gc map field from ArtMethod since we
should be able to get it from the Oat method header.

Bug: 17643507

Change-Id: Ie9508f05907a9f693882d4d32a564460bf273ee8