Add support for the rumprun unikernel

For most parts, rumprun currently looks like NetBSD, as they share the same
libc and drivers. However, being a unikernel, rumprun does not support
process management, signals or virtual memory, so related functions
might fail at runtime. Stack guards are disabled exactly for this reason.

Code for rumprun is always cross-compiled, it uses always static
linking and needs a custom linker.

src/libstd/sys/unix/stack_overflow.rs

@@ -34,7 +34,7 @@ impl Drop for Handler {
 #[cfg(any(target_os = "linux",
           target_os = "macos",
           target_os = "bitrig",
-          target_os = "netbsd",
+          all(target_os = "netbsd", not(target_vendor = "rumprun")),
           target_os = "openbsd"))]
 mod imp {
     use super::Handler;
@@ -143,7 +143,7 @@ mod imp {
 #[cfg(not(any(target_os = "linux",
               target_os = "macos",
               target_os = "bitrig",
-              target_os = "netbsd",
+              all(target_os = "netbsd", not(target_vendor = "rumprun")),
               target_os = "openbsd")))]
 mod imp {
     use ptr;