Fix glibc detection
Fix test data directory discovery on Linux
Remove assertions assuming overly deterministic dlmalloc behavior
Fix symbolutil and backtracer tests on Linux
Skip symbols without an address

This typically happens due to inlining.
Adjust glibc backtracer skip count
Initialize Interceptor's global state when initializing the library

This fixes the crash scenario where a backtrace is captured before the
Interceptor has been used for the first time.