arm: use correct argument size of special registers

Compiling with clang on ARMv8 shows errors like:

./arch/arm/include/asm/system.h:162:32: note: use constraint modifier "w"
                asm volatile("msr sctlr_el1, %0" : : "r" (val) : "cc");
                                             ^~
                                             %w0

These errors are due to using an incorrect size for the variables used
for writing to and reading from special registers which have 64 bits on
ARMv8.

Mask off reserved bits when reading the exception level.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>