Jit Code Cache instruction pipeline flushing

Restores instruction pipeline flushing on all cores following crashes
on ARMv7 with dual JIT code page mappings. We were inadvertantly
toggling permission on a non-executable page rather than executable.

Removes the data cache flush for roots data and replaces it with a
sequentially consistent barrier.

Fix MemMap::RemapAtEnd() when all pages are given out. To meet
invariants checked in the destructor, the base pointer needs to be
assigned as nullptr when this happens.

Bug: 63833411
Bug: 62332932
Test: art/test.py --target
Change-Id: I705cf5a3c80e78c4e912ea3d2c3c4aa89dee26bb