aarch64: add extra_context struct from linux v4.13

allows expanding the signal frame beyond the 4k reserved space.
new in linux commit 33f082614c3443d937f50fe936f284f62bbb4a1b

arch/aarch64/bits/signal.h

@@ -24,6 +24,7 @@ typedef struct sigcontext {
 
 #define FPSIMD_MAGIC 0x46508001
 #define ESR_MAGIC 0x45535201
+#define EXTRA_MAGIC 0x45585401
 struct _aarch64_ctx {
 	unsigned int magic;
 	unsigned int size;
@@ -38,6 +39,12 @@ struct esr_context {
 	struct _aarch64_ctx head;
 	unsigned long esr;
 };
+struct extra_context {
+	struct _aarch64_ctx head;
+	unsigned long datap;
+	unsigned int size;
+	unsigned int __reserved[3];
+};
 #else
 typedef struct {
 	long double __regs[18+256];