target-arm/cpu.h: Make cpu_init("nonexistent cpu") return NULL

target-arm/cpu.h: Make cpu_init("nonexistent cpu") return NULL

The macro definition of cpu_init meant that if cpu_arm_init()
returned NULL this wouldn't result in cpu_init() itself returning
NULL. This had the effect that "-cpu foo" for some unknown CPU
name 'foo' would cause ARM targets to segfault rather than
generating a useful error message. Fix this by making cpu_init
a simple inline function.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Andreas Färber <afaerber@suse.de>