selftests: sud_test: return correct emulated syscall value on RISC-V

Currently, the sud_test expects the emulated syscall to return the
emulated syscall number. This assumption only works on architectures
were the syscall calling convention use the same register for syscall
number/syscall return value. This is not the case for RISC-V and thus
the return value must be also emulated using the provided ucontext.

Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Link: https://lore.kernel.org/r/20231206134438.473166-1-cleger@rivosinc.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>