arm64: LLVMLinux: Use global stack pointer in return_address()

The global register current_stack_pointer holds the current stack pointer.
This change supports being able to compile the kernel with both gcc and clang.

Author: Mark Charlebois <charlebm@gmail.com>
Signed-off-by: Mark Charlebois <charlebm@gmail.com>
Signed-off-by: Behan Webster <behanw@converseincode.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>