TBR: i#2096: fix sysenter hook assert with new TLS scheme

Commit log for first patchset:
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i#2096: fix sysenter hook assert with new TLS scheme

We entering dispatch for the sysenter hook while native, and i#2089 made
get_thread_private_dcontext() return NULL while native.  We relax the
assert and fix up the KSTOP (xref i#2098).

Fixes #2096
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[bruening, 2016-12-11 00:48:46]

Committed as
https://github.com/DynamoRIO/dynamorio/commit/07765790951697e69f774a8c9421054...

Final commit log: 
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i#2096: fix sysenter hook assert with new TLS scheme

We entering dispatch for the sysenter hook while native, and i#2089 made
get_thread_private_dcontext() return NULL while native.  We relax the
assert and fix up the KSTOP (xref i#2098).

Fixes #2096

Review-URL: https://codereview.appspot.com/315960043
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