Fix register read callback in linux-arm single stepping

The previous read callback always read the value of the register what
caused problems when the emulator wrote some value into a register and
then expected to read the same value back. This CL add a register value
cache into the callbacks to return the correct value after a register
write also.

Test Plan: Stepping over BL/BLX instruction works on android-arm if the instruction set isn't change (other, unrelated patch will come for the case when we move to an other instruction set)

Reviewers: omjavaid, sas, clayborg

Reviewed By: clayborg

Subscribers: labath, tberghammer, rengolin, aemerson, lldb-commits

Differential Revision: http://reviews.llvm.org/D9187

From: Tamas Berghammer <tberghammer@google.com>
llvm-svn: 235852