arm64: LLVMLinux: Calculate current_thread_info from current_stack_pointer

arm64: LLVMLinux: Calculate current_thread_info from current_stack_pointer

Use the global current_stack_pointer to get the value of the stack pointer.
This change supports being able to compile the kernel with both gcc and clang.

Signed-off-by: Behan Webster <behanw@converseincode.com>
Signed-off-by: Mark Charlebois <charlebm@gmail.com>
Reviewed-by: Jan-Simon Möller <dl9pf@gmx.de>
Reviewed-by: Olof Johansson <olof@lixom.net>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
(cherry picked from commit 786248705ecf5290f26534e8eef62ba6dd63b806)
Signed-off-by: Mark Brown <broonie@kernel.org>