Add arm <sys/ucontext.h>.

Change-Id: I14d435f9e3e82f77d8a7e886ff88c18f5d09d14a

libc/include/sys/ucontext.h

@@ -36,7 +36,43 @@ __BEGIN_DECLS
 
 #if defined(__arm__)
 
-#error TODO
+enum {
+  REG_R0 = 0,
+  REG_R1,
+  REG_R2,
+  REG_R3,
+  REG_R4,
+  REG_R5,
+  REG_R6,
+  REG_R7,
+  REG_R8,
+  REG_R9,
+  REG_R10,
+  REG_R11,
+  REG_R12,
+  REG_R13,
+  REG_R14,
+  REG_R15,
+};
+
+#define NGREG 18 /* Like glibc. */
+
+typedef int greg_t;
+typedef greg_t gregset_t[NGREG];
+
+/* TODO: fpregset_t. */
+
+#include <asm/sigcontext.h>
+typedef struct sigcontext mcontext_t;
+
+typedef struct ucontext {
+  unsigned long uc_flags;
+  struct ucontext* uc_link;
+  stack_t uc_stack;
+  mcontext_t uc_mcontext;
+  sigset_t uc_sigmask;
+  /* TODO: uc_regspace */
+} ucontext_t;
 
 #elif defined(__arm64__)