Add the missing aarch64 user structs.

Bug: http://b/23377194
Change-Id: I7d36717e129d2e044dc2cf24de4227c3bcdd60ce
diff --git a/libc/include/sys/user.h b/libc/include/sys/user.h
index ce4c07c..3312981 100644
--- a/libc/include/sys/user.h
+++ b/libc/include/sys/user.h
@@ -232,7 +232,17 @@
 
 #elif defined(__aarch64__)
 
-// There are no user structures for 64 bit arm.
+struct user_regs_struct {
+  uint64_t regs[31];
+  uint64_t sp;
+  uint64_t pc;
+  uint64_t pstate;
+};
+struct user_fpsimd_struct {
+  __uint128_t vregs[32];
+  uint32_t fpsr;
+  uint32_t fpcr;
+};
 
 #else