compiler: use weak aliases to provide default definitions

This is simpler and more portable.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch_init.h

@@ -34,6 +34,6 @@ int tcg_available(void);
 int kvm_available(void);
 int xen_available(void);
 
-CpuDefinitionInfoList GCC_WEAK_DECL *arch_query_cpu_definitions(Error **errp);
+CpuDefinitionInfoList *arch_query_cpu_definitions(Error **errp);
 
 #endif

compiler.h

@@ -50,16 +50,13 @@
 #   define __printf__ __gnu_printf__
 #  endif
 # endif
-#if defined(_WIN32)
-#define GCC_WEAK __attribute__((weak))
-#define GCC_WEAK_DECL GCC_WEAK
-#else
-#define GCC_WEAK __attribute__((weak))
-#define GCC_WEAK_DECL
-#endif
+# define QEMU_WEAK_ALIAS(newname, oldname) \
+        typeof(oldname) newname __attribute__((weak, alias (#oldname)))
 #else
 #define GCC_ATTR /**/
 #define GCC_FMT_ATTR(n, m)
+#define QEMU_WEAK_ALIAS(newname, oldname) \
+        _Pragma("weak " #newname "=" #oldname)
 #endif
 
 #endif /* COMPILER_H */

qmp.c

@@ -466,11 +466,12 @@ DevicePropertyInfoList *qmp_device_list_properties(const char *typename,
     return prop_list;
 }
 
-CpuDefinitionInfoList GCC_WEAK *arch_query_cpu_definitions(Error **errp)
+static CpuDefinitionInfoList *default_arch_query_cpu_definitions(Error **errp)
 {
     error_set(errp, QERR_NOT_SUPPORTED);
     return NULL;
 }
+QEMU_WEAK_ALIAS(arch_query_cpu_definitions, default_arch_query_cpu_definitions);
 
 CpuDefinitionInfoList *qmp_query_cpu_definitions(Error **errp)
 {