[llvm-readobj] - Fix letters used for dumping section types in GNU style.

I've noticed that when we have all regular flags set, we print "WAEXMSILoGTx"
instead of "WAXMSILOGTCE" printed by GNU readelf.

It happens because:
1) We print SHF_EXCLUDE at the wrong place.
2) We do not recognize SHF_COMPRESSED, we print "x" instead of "C".
3) We print "o" instead of "O" for SHF_OS_NONCONFORMING.

This patch fixes differences and adds test cases.

Differential revision: https://reviews.llvm.org/D71418