Rationalize visibility.

If it's C or POSIX, it's in. If it's BSD or GNU, it's guarded by __USE_BSD
or __USE_GNU.

Bug: https://code.google.com/p/android/issues/detail?id=194631
Change-Id: Ife51a21c2b37b060db56780d29c929805b199cb6
diff --git a/libc/include/sys/ucontext.h b/libc/include/sys/ucontext.h
index 9e3c653..f252a37 100644
--- a/libc/include/sys/ucontext.h
+++ b/libc/include/sys/ucontext.h
@@ -70,9 +70,9 @@
   stack_t uc_stack;
   mcontext_t uc_mcontext;
   sigset_t uc_sigmask;
-  // Android has a wrong (smaller) sigset_t on ARM.
+  /* Android has a wrong (smaller) sigset_t on ARM. */
   uint32_t __padding_rt_sigset;
-  // The kernel adds extra padding after uc_sigmask to match glibc sigset_t on ARM.
+  /* The kernel adds extra padding after uc_sigmask to match glibc sigset_t on ARM. */
   char __padding[120];
   unsigned long uc_regspace[128] __attribute__((__aligned__(8)));
 } ucontext_t;
@@ -92,7 +92,7 @@
   struct ucontext *uc_link;
   stack_t uc_stack;
   sigset_t uc_sigmask;
-  // The kernel adds extra padding after uc_sigmask to match glibc sigset_t on ARM64.
+  /* The kernel adds extra padding after uc_sigmask to match glibc sigset_t on ARM64. */
   char __padding[128 - sizeof(sigset_t)];
   mcontext_t uc_mcontext;
 } ucontext_t;
@@ -157,7 +157,7 @@
   stack_t uc_stack;
   mcontext_t uc_mcontext;
   sigset_t uc_sigmask;
-  // Android has a wrong (smaller) sigset_t on x86.
+  /* Android has a wrong (smaller) sigset_t on x86. */
   uint32_t __padding_rt_sigset;
   struct _libc_fpstate __fpregs_mem;
 } ucontext_t;