arm: asm/system.h: mrc and mcr need .arm if __thumb2__ is not set

arm: asm/system.h: mrc and mcr need .arm if __thumb2__ is not set

The mcr and msr instructions are available in Thumb mode only if
Thumb2 is supported. Therefore, if __thumb2__ is not set, make
sure we switch to ARM mode by inserting a .arm directive in the
inline assembly.

Fixes LTO link errors with kirkwood platforms, triggered by a later
commit:

 tools/buildman/buildman -o /tmp/build -eP sheevaplug
 [...]
 {standard input}:24085: Error: selected processor does not support `mrc p15,0,r3,c1,c0,0' in Thumb mode

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>