Make pthread_attr_getstack__main_thread test native-bridge friendly

Detect stack mappings based on a pointer to the stack instead of
relying on "[stack]" label. This does not change the test but
makes it native-bridge friendly. In translated environment native
bridge implementation may have a separate stack for the guest code.

Bug: http://b/30443561
Test: bionic-unit-tests --gtest_filter=pthread*
Change-Id: Ic98a3b74b43ec63c7b37821a1f86525d32efd459