arm64: LLVMLinux: Use global stack register variable for aarch64
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 <f1891e7fc976ae1bd489627e2954fb730751d728@gmail.com>
Signed-off-by: Mark Charlebois <f1891e7fc976ae1bd489627e2954fb730751d728@gmail.com>
Signed-off-by: Behan Webster <9152da451d71c2a1ceb0c7b0908d4018fc127ae6@converseincode.com>
Signed-off-by: Will Deacon <b913f13ef92ac157955c954ccaaff7be065faba4@arm.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs