register_state_check.h: add compiler barrier

register_state_check.h: add compiler barrier

around ASM_REGISTER_STATE_CHECK() this helps keep the call ordering
consistent avoiding some code reordering which may affect the registers
being checked

fixes issue with armv7 and multiple versions of gcc:
[ RUN      ] C/AddNoiseTest.CheckNoiseAdded/0
test/register_state_check.h:116: Failure
Expected equality of these values:
  pre_store_[i]
    Which is: 0
  post_store[i]
    Which is: 4618441417868443648

Bug: webm:1760
Change-Id: Ib8bcefd2c4d263f9fc4d4b4d4ffb853fe89d1152
Fixed: webm:1760