[llvm-readobj] Display multiple function names for stack size entries

The current implementation of displaying .stack_size information
presumes that each entry represents a single function but this is not
always the case. For example with the use of ICF multiple functions can
be represented with the same code, meaning that the address found in a
.stack_size entry corresponds to multiple function symbols.
This change allows multiple function names to be displayed when
appropriate.

Differential Revision: https://reviews.llvm.org/D105884