Use PC-relative pointer to boot image methods.

In preparation for adding ArtMethod entries to the .bss
section, add direct PC-relative pointers to methods so that
the number of needed .bss entries for boot image is small.

Test: m test-art-host-gtest
Test: testrunner.py --host
Test: testrunner.py --target on Nexus 6P
Test: Nexus 6P boots.
Test: Build aosp_mips64-userdebug
Bug: 30627598
Change-Id: Ia89f5f9975b741ddac2816e1570077ba4b4c020f