[ASan] Disable strict init-order checking if dlopen() is called.

Revise the fix to https://code.google.com/p/address-sanitizer/issues/detail?id=178:
always disable strict init-order checking the first time dlopen() is
called: at this point shared library is allowed to access globals
defined in the main executable, as they are guaranteed to be
initialized. Revise the test cases:
* simplify init-order-dlopen.cc test case: make it Linux-specific
  (there's no strict init-order checking on other platforms anyway),
  and single-threaded.
* reinforce init-order-pthread-create.cc test case: make sure that
  init-order checker would produce a false positive unless we
  turn it off at the moment we call pthread_create().

llvm-svn: 230288