syscalls: compat: Fix the missing part for __SYSCALL_COMPAT

syscalls: compat: Fix the missing part for __SYSCALL_COMPAT

Make "uapi asm unistd.h" could be used for architectures' COMPAT
mode. The __SYSCALL_COMPAT is first used in riscv.

Signed-off-by: Guo Ren <[email protected]>
Signed-off-by: Guo Ren <[email protected]>
Reviewed-by: Arnd Bergmann <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Tested-by: Heiko Stuebner <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Palmer Dabbelt <[email protected]>