aarch64: Make elf_machine_{load_address,dynamic} robust [BZ #28203]

aarch64: Make elf_machine_{load_address,dynamic} robust [BZ #28203]

The AArch64 ABI is largely platform agnostic and does not specify
_GLOBAL_OFFSET_TABLE_[0] ([1]). glibc ld.so turns out to be probably the
only user of _GLOBAL_OFFSET_TABLE_[0] and GNU ld defines the value
to the link-time address _DYNAMIC. [2]

In 2012, __ehdr_start was implemented in GNU ld and gold in binutils
2.23.  Using adrp+add / (-mcmodel=tiny) adr to access
__ehdr_start/_DYNAMIC gives us a robust way to get the load address and
the link-time address of _DYNAMIC.

[1]: From a psABI maintainer, https://bugs.llvm.org/show_bug.cgi?id=49672#c2
[2]: LLD's aarch64 port does not set _GLOBAL_OFFSET_TABLE_[0] to the
link-time address _DYNAMIC.
LLD is widely used on aarch64 Android and ChromeOS devices.  Software
just works without the need for _GLOBAL_OFFSET_TABLE_[0].

Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>