arch/csky: move GCC_TARGET_CPU calculation to arch.mk.csky

Calculating GCC_TARGET_CPU requires combining multiple flags, which
isn't very nicely expressed in Config.in, so let's move this into
arch.mk.csky, similarly to what is done in arch.mk.riscv.

Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
[Thomas: rework commit log]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>