Avoid sign extension in packed-switch.

Avoid sign extension in packed-switch.

This code (at least in the ARM version) is trying to assign to r0 and r1
from C by returning a 64-bit result. The mistaken use of signed integers
for pointers can lead to sign extension if the JIT code cache is at a
sufficiently high address.

Bug: 6799823
Bug: 6703991

(cherry picked from commit 2d0c1c2dbe44458ebb199c47ce1047f266db5349)

Conflicts:

	vm/compiler/codegen/mips/CalloutHelper.h
	vm/compiler/codegen/mips/CodegenDriver.cpp

Change-Id: Id4699fdd0b3d61abe9c92874832b8561df4fa797