runtime: populate signal PC on NetBSD

The NetBSD libc provides an architecture-independent macro that can
extract the PC from a ucontext struct.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/261740

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-5e76d81ec120e05a59e6c7d173ddf8a3de466bd0
+6cb7b9e924d84125f21f4a2a96aa0d59466056fe
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/runtime/go-signal.c

@@ -229,6 +229,8 @@ getSiginfo(siginfo_t *info, void *context __attribute__((unused)))
 	ret.sigpc = ((ucontext_t*)(context))->uc_mcontext.jmp_context.iar;
 #elif defined(__aarch64__) && defined(__linux__)
 	ret.sigpc = ((ucontext_t*)(context))->uc_mcontext.pc;
+#elif defined(__NetBSD__)
+	ret.sigpc = _UC_MACHINE_PC(((ucontext_t*)(context)));
 #endif
 
 	if (ret.sigpc == 0) {