riscv64: typedef struct ucontext {...} ucontext_t.

All the other architectures are already polluting the namespace with
`struct ucontext`, so make riscv64 match for source compatibility with
other Android code. (Code _should_ be using the POSIX `ucontext_t`, but
ART in particular had a lot of `struct ucontext`, and although I'll
clean that up separately, if there's some in our tree, there's probably
a lot more out there in the wild.)

Test: treehugger
Change-Id: Id0e4e97e660d7d60e792cd2462ddb9788d4772d7
diff --git a/libc/include/sys/ucontext.h b/libc/include/sys/ucontext.h
index cefe177..bb6443b 100644
--- a/libc/include/sys/ucontext.h
+++ b/libc/include/sys/ucontext.h
@@ -366,9 +366,9 @@
 
 /* This matches the kernel <asm/ucontext.h> but using mcontext_t. */
 
-typedef struct ucontext_t {
+typedef struct ucontext {
   unsigned long uc_flags;
-  struct ucontext_t* uc_link;
+  struct ucontext* uc_link;
   stack_t uc_stack;
   union {
     sigset_t uc_sigmask;