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: NMark Charlebois <charlebm@gmail.com>
Signed-off-by: NBehan Webster <behanw@converseincode.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>