The CPP builtins execute the same piece of code to prepare context before
jumping into CEntryStub. By creating new ASM builtin to execute that common
piece of code, ~7KB code size (tested on x64) of snapshot_blob.bin can be 
reduced without any negative performance impact.

BUG=

Change-Id: I744369e8723dcd902b61dc50645db66bea884441
Reviewed-on: https://chromium-review.googlesource.com/595119Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47590}