Add greg_t for arm64.

This was already present for the other architectures. I think we skipped
this because glibc seems to have an incorrect definition (int rather than
long), but the kernel has the sane definition (just not in a uapi header).

(cherry picked from commit 8e4d371091e5738346f5c6ad395b8487c2a5ec67)

Bug: 18172268
Change-Id: I22d13fdeb6431ea122dd028a229782dcaf2286b2

libc/include/sys/ucontext.h

@@ -78,6 +78,10 @@ typedef struct ucontext {
 
 #elif defined(__aarch64__)
 
+#define NGREG 34 /* x0..x30 + sp + pc + pstate */
+typedef unsigned long greg_t;
+typedef greg_t gregset_t[NGREG];
+
 #include <asm/sigcontext.h>
 typedef struct sigcontext mcontext_t;