[llvm-readobj] Optimize printing stack sizes to linear time.

Currently, each function name lookup is a linear iteration over all symbols defined in the object file which makes the total running time quadratic.

This patch optimizes the function name lookup by populating an **address to index** map upon the first function name lookup which is used to lookup each function name in O(1).

**impact**: For the clang binary built with `-fstack-size-section`, this improves the running time of `llvm-readobj --stack-size` from 7 minutes to 0.25 seconds.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D103072