Use __ehdr_start rather than _begin in _dl_start_final

Use __ehdr_start rather than _begin in _dl_start_final

__ehdr_start is already used in rltld.c:dl_main, and can serve the
same purpose as _begin.  Besides tidying the code, using linker
defined section relative symbols rather than "-defsym _begin=0" better
reflects the intent of _dl_start_final use of _begin, which is to
refer to the load address of ld.so rather than absolute address zero.

Reviewed-by: Florian Weimer <fweimer@redhat.com>