Add very simple loading test for non-SFI NaCl.

This is the first step to add test cases for non-SFI NaCl.
The testee binary is build by a new target rule introduced by this CL, which is relocatable/libc-free binary. The code has a minimum set of handlers for irt_ppapi, to load the plugin successfully done with communicating hosts (browser/renderer processes).

Along this change, some DCHECK errors are found. So they're fixed, too.
1) AtExitManager is conflicting. In non-SFI mode, AtExitManager is already instantiated in nacl_helper. To avoid the conflicting, skip to create it in plugin_main_irt.cc.
2) Allowed creating Singleton on the plugin's main thread.
3) SRPC may be successfully done, and then NOTREACHED catch it in nacl_listener. It is false alarm, so removed it.

BUG=https://code.google.com/p/nativeclient/issues/detail?id=3734
TEST=run ./browser_tests --gtest_filter=NaCl*NonSFI* locally. Run trybots.

Review URL: https://codereview.chromium.org/189553005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@257118 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 257118