Use hexadecimal notation for section header flags in case we have a section header with flag without a textual/symbolic representation. The hex syntax is introduced in GNU as version 2.27  and was added to llvm in r289785 . A previous change to do was rejected  and this change takes the comments there into account.
I've refactored the section header flags printing to detect if there are any flags we have no symbolic section flag for. In such case, print the section header flags as hex instead.