[llvm-readobj][ELF] Teach llvm-readobj to show arch specific ELF section's flags

Some architecture specific ELF section flags might have the same value
(for example SHF_X86_64_LARGE and SHF_HEX_GPREL) and we have to check
machine architectures to select an appropriate set of possible flags.

The patch selects architecture specific flags into separate arrays
`ElfxxxSectionFlags` and combines `ElfSectionFlags` and `ElfxxxSectionFlags`
before pass to the `StreamWriter::printFlags()` method.

Differential Revision: http://reviews.llvm.org/D16269

llvm-svn: 258334