cpu: add CPU_RESOLVING_TYPE macro

it will be used for providing to cpu name resolving class for
parsing cpu model for system and user emulation code.

Along with change add target to null-machine tests, so
that when switch to CPU_RESOLVING_TYPE happens,
it would ensure that null-machine usecase still works.

Backports commit 0dacec874fa3b3fd34b0d0670fa257efdcbbebd0 from qemu

qemu/target/arm/cpu.h

@@ -2273,6 +2273,7 @@ static inline bool arm_excp_unmasked(CPUState *cs, unsigned int excp_idx,
 
 #define ARM_CPU_TYPE_SUFFIX "-" TYPE_ARM_CPU
 #define ARM_CPU_TYPE_NAME(name) (name ARM_CPU_TYPE_SUFFIX)
+#define CPU_RESOLVING_TYPE TYPE_ARM_CPU
 
 #define cpu_signal_handler cpu_arm_signal_handler
 #define cpu_list arm_cpu_list

qemu/target/i386/cpu.h

@@ -1525,6 +1525,7 @@ uint64_t cpu_get_tsc(CPUX86State *env);
 
 #define X86_CPU_TYPE_SUFFIX "-" TYPE_X86_CPU
 #define X86_CPU_TYPE_NAME(name) (name X86_CPU_TYPE_SUFFIX)
+#define CPU_RESOLVING_TYPE TYPE_X86_CPU
 
 #define cpu_signal_handler cpu_x86_signal_handler
 #endif

qemu/target/m68k/cpu.h

@@ -525,6 +525,7 @@ enum {
 
 #define M68K_CPU_TYPE_SUFFIX "-" TYPE_M68K_CPU
 #define M68K_CPU_TYPE_NAME(model) model M68K_CPU_TYPE_SUFFIX
+#define CPU_RESOLVING_TYPE TYPE_M68K_CPU
 
 #define cpu_signal_handler cpu_m68k_signal_handler
 #define cpu_list m68k_cpu_list

qemu/target/mips/cpu.h

@@ -747,6 +747,7 @@ enum {
 
 #define MIPS_CPU_TYPE_SUFFIX "-" TYPE_MIPS_CPU
 #define MIPS_CPU_TYPE_NAME(model) model MIPS_CPU_TYPE_SUFFIX
+#define CPU_RESOLVING_TYPE TYPE_MIPS_CPU
 
 int cpu_mips_signal_handler(int host_signum, void *pinfo, void *puc);
 bool cpu_supports_isa(struct uc_struct *uc, const char *cpu_model, unsigned int isa);

qemu/target/sparc/cpu.h

@@ -647,6 +647,7 @@ int cpu_sparc_signal_handler(int host_signum, void *pinfo, void *puc);
 
 #define SPARC_CPU_TYPE_SUFFIX "-" TYPE_SPARC_CPU
 #define SPARC_CPU_TYPE_NAME(model) model SPARC_CPU_TYPE_SUFFIX
+#define CPU_RESOLVING_TYPE TYPE_SPARC_CPU
 
 #define cpu_signal_handler cpu_sparc_signal_handler
 #define cpu_list sparc_cpu_list