[ELF] Use pcrel format for eh_frame_ptr field encoding

The `eh_frame_ptr` field in the `.eh_frame_hdr` section contains an address
of the `.eh_frame` section. Using an absolute 32-bit format for encoding
of this field does not work for 64-bit targets. It is better to use a
relative format because it covers both 32-bit and 64-bit cases. Sure
this work if a distance between `.eh_frame_hdr` and `.eh_frame` sections
is less than 4 Gb but it is a rather correct assumption.

http://reviews.llvm.org/D8352

llvm-svn: 232414