Move caller-saves saving/restoring to ReadBarrierMarkRegX.

Instead of saving/restoring live caller-save registers
before/after the call to read barrier mark entry points
ReadBarrierMarkRegX, have these entry points save/restore
all the caller-save registers themselves (except register
rX, which contains the return value).

Also refactor the assembly code of these entry points
using macros.

* Boot image code size variation on Nexus 5X
  (aosp_bullhead-userdebug build):
  - total ARM64 framework Oat files size change:
    119196792 bytes -> 115575920 bytes (-3.04%)
  - total ARM framework Oat files size change:
    100435212 bytes -> 97621188 bytes (-2.80%)

* Benchmarks (ARM64) score variations on Nexus 5X
  (aosp_bullhead-userdebug build):
  - RitzPerf (lower is better)
    - average score difference: -2.71%
  - CaffeineMark (higher is better)
    - no real difference for most tests
      (absolute variation lower than 1%)
    - better score on the "Method" benchmark:
      score variation 41253 -> 44891 (+8.82%)

Test: ART host and target (ARM, ARM64) tests.
Bug: 29506760
Bug: 12687968
Change-Id: I881bf73139a3f1c2bee9ffc6fc8c00f9a392afa6

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