UPSTREAM: arm64: LLVMLinux: Add current_stack_pointer() for arm64

Define a global named register for current_stack_pointer. The use of this new
variable guarantees that both gcc and clang can access this register in C code.

Signed-off-by: Behan Webster <behanw@converseincode.com>
Reviewed-by: Jan-Simon Möller <dl9pf@gmx.de>
Reviewed-by: Mark Charlebois <charlebm@gmail.com>
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 3337a10e0d0cbc9225cefc23aa7a604b698367ed)
Bug: 29520177
Signed-off-by: Mohan Srinivasan <srmohan@google.com>
Change-Id: I19acaf0740ed061d47e4d4c6547142d45aaf81d4