makefile: Optimized the generation mechanism of kernel header files

[cfc4n]

Run the make prepare command only if the arch/$(LINUX_ARCH)/kernel/asm-offsets.s file does not exist. This is because this file is generated during the make phase.

CROSS_ARCH=amd64 (on arm64 ubuntu 22.04)

  CC      kernel/bounds.s
  CC      arch/x  86/kernel/asm-offsets.s
  CALL    scripts/checksyscalls.sh
  CALL    scripts/atomic/check-atomics.sh

arm64(aarch64)

  CC      kernel/bounds.s
  UPD     include/generated/bounds.h
  CC      arch/arm64/kernel/asm-offsets.s
  UPD     include/generated/asm-offsets.h
  CALL    scripts/checksyscalls.sh
  CALL    scripts/atomic/check-atomics.sh