Fix x86 & x86-64 UnsafeGetObject intrinsics with read barriers.

The implementation was incorrectly interpreting the 'offset'
input as an index in a (4-byte) object reference array,
whereas it is a (1-byte) offset to an object reference field
within the 'base' (object) input.

Bug: 29516905
Change-Id: Idfbead8289222b55069816a81284401eff791e85
Test: Covered by test/004-UnsafeTest.
4 files changed