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.

configure

@@ -1295,6 +1295,12 @@ $ pacman -R cmake && pacman -S mingw-w64-x86_64-cmake
             putvar CFG_MSVC_LIB_PATH_${bits}
             ;;
 
+        *-rumprun-netbsd)
+            step_msg "targeting rumprun-netbsd, disabling jemalloc"
+            CFG_DISABLE_JEMALLOC=1
+            putvar CFG_DISABLE_JEMALLOC
+            ;;
+
         *)
             ;;
     esac