[llvm-readobj/elf] - Cleanup helpers that are used to print segment types. NFCI.

`getElfSegmentType` and `getElfPtType` are methods that are used for printing
segment types for LLVM and GNU styles accordingly.

This patch does a cleanup and simplification that allows to avoid
the code duplication and to get rid of one macro.

Differential revision: https://reviews.llvm.org/D85830