arm64: LLVMLinux: Use global stack register variable for aarch64

To support both Clang and GCC, use the global stack register variable vs
a local register variable.

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>