Makefile: ARMv7-M has no CPSR register

Makefile: ARMv7-M has no CPSR register

Compiling on ARMv7-M fails when trying to address the CPSR register which
is not available on this architecture.

Atomic functions refer to the CPSR register if compiled with
arch/arm/include/asm/proc-armv/system.h. On ARMv7-M we should hence
use arch/arm/thumb1/include/asm/proc-armv/system.h instead.

Cf. https://stackoverflow.com/questions/61097841/error-selected-processor-does-not-support-requested-special-purpose-register

Reported-by: Sicris Rey Embay <sicris.embay@gmail.com>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>