* New atomic code stubs for x64, ia32, arm, arm64
* Add convenience functions JumpIfNotValidSmiValue, JumpIfUintNotValidSmiValue
  to macro-assembler-ia32 (API based on x64 macro assembler)
* Remove runtime implementation of Atomics.load, the code stub should always be
  called instead
* Add new test to mjsunit atomics test; check that Smi values of different
  sizes are supported when possible, else fall back to HeapNumbers

These changes were needed to add another codestub:
* Bump kStubMajorKeyBits from 7 to 8
* Reduce ScriptContextFieldStub::kSlotIndexBits from 13 to 12

BUG=v8:4614
LOG=y

Review URL: https://codereview.chromium.org/1617503003

Cr-Commit-Position: refs/heads/master@{#35427}