riscv32: Fix `struct ucontext` definition.

The correct typedef is `struct ucontext` -> `ucontext_t`,
not `struct ucontext_t` -> `ucontext_t` like for other architectures.

Signed-off-by: Dmitry Chestnykh <dm.chestnykh@gmail.com>

libc/sysdeps/linux/riscv32/sys/ucontext.h

@@ -83,10 +83,10 @@ typedef struct mcontext_t
   } mcontext_t;
 
 /* Userlevel context.  */
-typedef struct ucontext_t
+typedef struct ucontext
   {
     unsigned long int  __uc_flags;
-    struct ucontext_t *uc_link;
+    struct ucontext   *uc_link;
     stack_t            uc_stack;
     sigset_t           uc_sigmask;
     /* There's some padding here to allow sigset_t to be expanded in the