Adds initial skeletal COMPAT Kbuild (Running 32bit U-mode on
64bit S-mode) support.
 - Setup kconfig & dummy functions for compiling.
 - Implement compat_start_thread by the way.
Signed-off-by: NGuo Ren <guoren@linux.alibaba.com>
Signed-off-by: NGuo Ren <guoren@kernel.org>
Reviewed-by: NArnd Bergmann <arnd@arndb.de>
Tested-by: NHeiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20220405071314.3225832-21-guoren@kernel.orgSigned-off-by: NPalmer Dabbelt <palmer@rivosinc.com>