v8: fix openbsd build

This is [1] applied ahead of time. Summary:

    OpenBSD doesn't have <ucontext.h>. ucontext_t lives in <signal.h>
    and is a typedef for struct sigcontext. There is no uc_mcontext.

[1] https://codereview.chromium.org/21705003/

deps/v8/src/sampler.cc

@@ -38,7 +38,10 @@
 #include <signal.h>
 #include <sys/time.h>
 #include <sys/syscall.h>
-#if !defined(__ANDROID__) || defined(__BIONIC_HAVE_UCONTEXT_T)
+// OpenBSD doesn't have <ucontext.h>. ucontext_t lives in <signal.h>
+// and is a typedef for struct sigcontext. There is no uc_mcontext.
+#if (!defined(__ANDROID__) || defined(__BIONIC_HAVE_UCONTEXT_T)) \
+    && !defined(__OpenBSD__)
 #include <ucontext.h>
 #endif
 #include <unistd.h>
@@ -330,7 +333,9 @@ void SignalHandler::HandleProfilerSignal(int signal, siginfo_t* info,
 #else
   // Extracting the sample from the context is extremely machine dependent.
   ucontext_t* ucontext = reinterpret_cast<ucontext_t*>(context);
+#if !defined(__OpenBSD__)
   mcontext_t& mcontext = ucontext->uc_mcontext;
+#endif
 #if defined(__linux__) || defined(__ANDROID__)
 #if V8_HOST_ARCH_IA32
   state.pc = reinterpret_cast<Address>(mcontext.gregs[REG_EIP]);
@@ -384,7 +389,6 @@ void SignalHandler::HandleProfilerSignal(int signal, siginfo_t* info,
   state.fp = reinterpret_cast<Address>(mcontext.__gregs[_REG_RBP]);
 #endif  // V8_HOST_ARCH_*
 #elif defined(__OpenBSD__)
-  USE(mcontext);
 #if V8_HOST_ARCH_IA32
   state.pc = reinterpret_cast<Address>(ucontext->sc_eip);
   state.sp = reinterpret_cast<Address>(ucontext->sc_esp);