x86-64: Allow loads from some structs/unions

GCC allows register loads for asms based on type mode,
and correctly sized structs/union have an allowed mode
(basically 1,2,4,8 sized aggregates).

tests/tcctest.c

@@ -2677,6 +2677,7 @@ void asm_test(void)
     struct struct1231 s2 = { (unsigned long)&s1 };
     /* Hide the outer base_func, but check later that the inline
        asm block gets the outer one.  */
+    int base_func = 42;
     void override_func3 (void);
     unsigned long asmret;
 
@@ -2720,6 +2721,11 @@ void asm_test(void)
     override_func2();
     /* The base_func ref from the following inline asm should find
        the global one, not the local decl from this function.  */
+    asm volatile(".weak override_func3\n.set override_func3, base_func");
+    override_func3();
+    /* Check that we can also load structs of appropriate layout
+       into registers.  */
+    asm volatile("" : "=r" (asmret) : "0"(s2));
     if (asmret != s2.addr)
       printf("asmstr: failed\n");
     return;