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.

run-gdb

@@ -152,12 +152,8 @@ See: https://github.com/cirosantilli/linux-kernel-module-cheat#gdb-builtin-cpu-s
             test_script_path = os.path.splitext(self.env['source_path'])[0] + '.py'
         else:
             image = self.env['vmlinux']
-        if self.env['baremetal']:
-            allowed_toolchains = ['crosstool-ng', 'buildroot', 'host']
-        else:
-            allowed_toolchains = ['buildroot', 'crosstool-ng', 'host']
         cmd = (
-            [self.get_toolchain_tool('gdb', allowed_toolchains=allowed_toolchains), LF] +
+            [self.get_toolchain_tool('gdb'), LF] +
             before
         )
         if linux_full_system: