ARM: Use rMR for Baker RB introspection marking.

The marking register (r8 on ARM) is known to be 1 when
entering the introspection marking entrypoint, so we can
clobber it, use it as a temporaray register (instead or r4)
in the runtime entrypoint, and reload the 1 before
returning. The immediate benefits are minor, see below,
but this shall allow further improvements, for example we
could try to change rMR to r4 which would reduce code size
of every marking register check by 2 bytes.

ARM boot image (boot*.oat) size in aosp_taimen-userdebug:
  - before: 17861724
  - after: 17858088 (-3636)

Test: Pixel 2 XL boots.
Test: m test-art-host-gtest
Test: testrunner.py --target --optimizing --32
Test: Repeat the above tests with heap poisoning enabled.
Bug: 36141117
Change-Id: I0f625dec3a6b3ee1786f7e5f4377be42b9bc37d3

0 files changed, 0 insertions, 0 deletions