ARM: Use r4 for stack overflow check to reduce code size.

The code savings are 2 bytes per stack overflow check but
this can be rounded up or down to the kArmAlignment (8).
Current testing shows the boot image size difference for
aosp_taimen-userdebug arm boot*.oat as
  - before: 17939352
  - after: 17881764 (-56KiB, -0.3%)

Test: Pixel 2 XL boots.
Test: testrunner.py --target -t 018-stack-overflow
Bug: 71627785
Change-Id: Icb732b59e9e681b29790e7e07de2710da33245b1