[llvm-readelf] - Change letters used for SHF_ARM_PURECODE and SHF_X86_64_LARGE flags.

GNU uses `l` for SHF_X86_64_LARGE and `y` for SHF_ARM_PURECODE.
Lets follow.

To do this I had to refactor and refine how we print the help flags description.
It was too generic and inconsistent with GNU readelf.

Differential revision: https://reviews.llvm.org/D71464