[llvm-readobj] Print ELF header flags names in GNU output

GNU readelf tool prints hex value of the ELF header flags field and the
flags names. This change adds the same functionality to llvm-readobj.
Now llvm-readobj can print MIPS and RISCV flags.

New GNUStyle::printFlags() method is a copy of ScopedPrinter::printFlags()
routine. Probably we can escape code duplication and / or simplify the
printFlags() method. But it's a task for separate commit.

Differential revision: https://reviews.llvm.org/D52027

llvm-svn: 345238