riscv: sigcontext: Correct the comment of sigreturn

riscv: sigcontext: Correct the comment of sigreturn

The real-time signals enlarged the sigset_t type, and most architectures
have changed to using rt_sigreturn as the only way. The riscv is one of
them, and there is no sys_sigreturn in it. Only some old architecture
preserved sys_sigreturn as part of the historical burden.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>
Link: https://lore.kernel.org/r/20230628091213.2908149-1-guoren@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>