ARM: Add r3 to clobber list when configuring stack guard

If you enable HW_STACK_PROTECTION, DEBUG_OPTIMIZATIONS, and
CPU_STATS, you can get a crash when switching to the main thread
due to the r3 getting munged during a inline ASM call.  This patch
fixes that by adding r3 to the clobber list.

Fixes #12821

Signed-off-by: Andy Gross <andy.gross@linaro.org>