x86_64: Decouple return types from FFI_TYPE constants

We can better support structure returns, and as prep for
complex types.

src/x86/internal64.h

@@ -0,0 +1,20 @@
+#define UNIX64_RET_VOID		0
+#define UNIX64_RET_UINT8	1
+#define UNIX64_RET_UINT16	2
+#define UNIX64_RET_UINT32	3
+#define UNIX64_RET_SINT8	4
+#define UNIX64_RET_SINT16	5
+#define UNIX64_RET_SINT32	6
+#define UNIX64_RET_INT64	7
+#define UNIX64_RET_XMM32	8
+#define UNIX64_RET_XMM64	9
+#define UNIX64_RET_X87		10
+#define UNIX64_RET_ST_RAX_RDX	11
+#define UNIX64_RET_ST_XMM0_RAX	12
+#define UNIX64_RET_ST_RAX_XMM0	13
+#define UNIX64_RET_ST_XMM0_XMM1	14
+#define UNIX64_RET_LAST		14
+
+#define UNIX64_FLAG_RET_IN_MEM	(1 << 10)
+#define UNIX64_FLAG_XMM_ARGS	(1 << 11)
+#define UNIX64_SIZE_SHIFT	12