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> (cherry picked from commit 43d06ed2)
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