arm: Simplify elf_machine_{load_address,dynamic}

and drop reliance on _GLOBAL_OFFSET_TABLE_[0] being the link-time
address of _DYNAMIC. &__ehdr_start is a better way to get the load address.

This is similar to commits b37b75d2
(x86-64) and 43d06ed2 (aarch64).

Reviewed-by: Joseph Myers <joseph@codesourcery.com>