[Support/ELF] - Added few constants and structs relative to compressed sections.

Patch adds few constants and structs to support compressed sections.

SHF_COMPRESSED intersects with platform specific XCORE_SHF_CP_SECTION,
both has value of 0x800U.

Reference link:
http://www.sco.com/developers/gabi/latest/ch4.sheader.html

Differential revision: http://reviews.llvm.org/D20209

llvm-svn: 269404