Fixes for the profiler on arm64.

Fixes for the profiler on arm64.

On arm64, in Dart code, R18(SP) is the stack pointer.
In C++ code, R31(CSP) is the stack pointer.
The profiler must choose the right one when performing
its bounds checks.

This change also fixes a bug in the InvokeDartCode stub
on arm64 so that CSP is set to the Isolate's stack
limit immediately, rather than a bit later. When it was
set a bit later, if a profiler interrupt came in in the
interim, the stack would be smashed.

R=johnmccutchan@google.com

Committed: https://code.google.com/p/dart/source/detail?r=40502

[zra, 2014-09-18 16:47:22]

zra@google.com changed reviewers:
+ johnmccutchan@google.com

[zra, 2014-09-18 16:47:23]

Not sure I've got the right check in the profiler to choose the right sp. PTAL.

[Cutch, 2014-09-18 17:21:15]

lgtm

[zra, 2014-09-19 16:25:22]

Committed patchset #3 (id:40001) manually as r40502 (presubmit successful).