build: use just gcc instead of x86_64-linux-gnu-gcc when host arch == target arch and --gcc-which host

This is particularly important to easily build userland content on host,
as different distros will have different prefixes as mentioned at:
https://github.com/cirosantilli/linux-kernel-module-cheat/issues/66

build-modules

@@ -102,7 +102,7 @@ Place the modules on a separate magic directory from non --host builds.
                     'ARCH={}'.format(self.env['linux_arch']), LF,
                     'CC={}'.format(cc), LF,
                     'CCFLAGS={}'.format(self.sh.cmd_to_string(ccflags)), LF,
-                    'CROSS_COMPILE={}-'.format(self.env['toolchain_prefix']), LF,
+                    'CROSS_COMPILE={}'.format(self.env['toolchain_prefix_dash']), LF,
                     'LINUX_DIR={}'.format(linux_dir), LF,
                     'M={}'.format(build_subdir), LF,
                     'OBJECT_FILES={}'.format(' '.join(object_files)), LF,