[ARM] Generation of .ARM.exidx/.ARM.extab sections

This patch provides generation of .ARM.exidx & .ARM.extab sections which are
used for unwinding. The patch adds new content type typeARMExidx for atoms from
.ARM.exidx section and integration of atoms with such type to the ELF
ReaderWriter. exidx.test has been added with checking of contents of .ARM.exidx
section and .ARM.extab section.

Differential Revision: http://reviews.llvm.org/D9324

llvm-svn: 236873