symbolizer: Handle addresses with many symbols

[javierhonduco]

Symbolization was completely broken if inline functions were present because the code assumed that every program counter maps to exactly one symbol, but that's not true.

We didn't see this often because some applications we ran had no inlined functions, but it was very apparent with Ruby and Fortran code, especially with O2 and O3, as the compiler will start inlining at these levels more.

Test Plan

Tested with basic_f77_stack_O2.

before

after

[javierhonduco]

cc @gmarler who reported this issue!