target: Use forward declared type instead of structure type

The CPU / CPU state are forward declared.

  $ git grep -E 'struct [A-Za-z]+CPU\ \*'
  target/arm/hvf_arm.h:16:void hvf_arm_set_cpu_features_from_host(struct ARMCPU *cpu);
  target/openrisc/cpu.h:234:    int (*cpu_openrisc_map_address_code)(struct OpenRISCCPU *cpu,
  target/openrisc/cpu.h:238:    int (*cpu_openrisc_map_address_data)(struct OpenRISCCPU *cpu,

  $ git grep -E 'struct CPU[A-Za-z0-9]+State\ \*'
  target/mips/internal.h:137:    int (*map_address)(struct CPUMIPSState *env, hwaddr *physical, int *prot,
  target/mips/internal.h:139:    void (*helper_tlbwi)(struct CPUMIPSState *env);
  target/mips/internal.h:140:    void (*helper_tlbwr)(struct CPUMIPSState *env);
  target/mips/internal.h:141:    void (*helper_tlbp)(struct CPUMIPSState *env);
  target/mips/internal.h:142:    void (*helper_tlbr)(struct CPUMIPSState *env);
  target/mips/internal.h:143:    void (*helper_tlbinv)(struct CPUMIPSState *env);
  target/mips/internal.h:144:    void (*helper_tlbinvf)(struct CPUMIPSState *env);
  target/xtensa/cpu.h:347:    struct CPUXtensaState *env;
  ...

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220214183144.27402-12-f4bug@amsat.org>

target/arm/hvf_arm.h

@@ -13,6 +13,6 @@
 
 #include "cpu.h"
 
-void hvf_arm_set_cpu_features_from_host(struct ARMCPU *cpu);
+void hvf_arm_set_cpu_features_from_host(ARMCPU *cpu);
 
 #endif