Let arm_section_length store the number of bytes.

Exception section data that we extract for DWARF gets stored as the
offset and the number of bytes. For ARM exception info, we seem to
deviate from this by storing the number of entries. Attempt to make this
more consistent.

By storing the number of bytes, we can get rid of the EHTEntry structure
declared in AddressSpace.hpp. In UnwindCursor.hpp we already have
another structure declared for the same purpose.

Reviewed by:	Keith Walker
Differential Revision:	https://reviews.llvm.org/D30681

llvm-svn: 297149