[llvm-readobj] - Teach readobj to recognize SHF_COMPRESSED flag.

Main problem here was that SHF_COMPRESSED has the same value with
XCORE_SHF_CP_SECTION, which was included as standart (common) flag.
As far I understand xCore is a family of controllers and it that
means it's constant should be processed separately,
only if e_machine == EM_XCORE, otherwise llvm-readobj would output
different constants twice for compressed section:

Flags [
..
SHF_COMPRESSED (0x800)
..
XCORE_SHF_CP_SECTION (0x800)
..
]

what probably does not make sence if you're not working with xcore file.

Differential revision: http://reviews.llvm.org/D20273

llvm-svn: 270320