split --prebuilt and --host into --gcc-which and --qemu-which

Only one --host exists at ./build-modules, since that can select the host
kernel, which is independent from the toolchain.

Document that user mode simulation stopped working.

userland/uname.c

@@ -0,0 +1,20 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#fatal-kernel-too-old */
+
+#define _XOPEN_SOURCE 700
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/utsname.h>
+
+int main(void) {
+    struct utsname info;
+    if (uname(&info) == -1) {
+        perror("uname");
+        exit(EXIT_FAILURE);
+    }
+    printf("sysname  = %s\n", info.sysname );
+    printf("nodename = %s\n", info.nodename);
+    printf("release  = %s\n", info.release );
+    printf("version  = %s\n", info.version );
+    printf("machine  = %s\n", info.machine );
+    return EXIT_SUCCESS;
+}