Port b89d4249

Original Commit Message:

    This migrates the JSEntryStub to three dedicated builtins:

    JSEntry
    JSConstructEntry
    JSRunMicrotasksEntry

    correct macro assembler method to load and store external references
    through the kRootRegister).
    kRootRegister is initialized, so we first reserve the slot and later
    load its value.

R=jgruber@chromium.org, joransiu@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=
LOG=N

Change-Id: Ib84feca5e88d032307b5fab42377c56d074faf7f
Reviewed-on: https://chromium-review.googlesource.com/c/1393296Reviewed-by: Joran Siu <joransiu@ca.ibm.com>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#58503}