Fix ARM program header values used for exidx.

Before, I was using p_vaddr to get the offset into the elf file where
the exidx frame starts. I changed that to use p_offset since this already
has the load bias offset in it and some elf files do not set p_vaddr
properly.

Also, use p_filesz instead of p_memsz, since again, some elf files do
not set p_memsz to the same as p_filesz.

Bug: 110704153

Test: All libbacktrace/libunwindstack unit tests pass.
Test: Randomly unwind process on a walleye.
Test: Verified that this properly dumps and unwinds the shared
Test: library that sets p_vaddr and p_memsz differently.
Change-Id: Ic7b1e5d07439f4636fa02cd884a8727a5737372b