Enable weak functions properly.

There is a check for WEAKS_AVAILABLE that is never set. Change this so
so it is available everywhere except apple/windows.

Add new test to verify that on Android the logStackInternal function is
defined.

Bug: 331489939

Test: All unit tests pass.
Change-Id: Ie982eba30ae421931a62718d39ff1f0c282453db
2 files changed