loader: stop relying on AT_BASE

android loader should not rely on the fact
that AT_BASE is set because kernel currently
does not set it when linker is run standalone
(The linker does not have PT_INTERP set)

This commit replaces AT_BASE with calculated value.

Bug: http://b/30739481
Bug: http://b/35890756
Test: run bionic-unit-tests --gtest_filter=dl*
Change-Id: Ic2eb73e4452624b1f2e05f46e99e4c17df0bbc3f