[PATCH 2.27] RISC-V: fix struct kernel_sigaction to match the kernel version [BZ #23069]

[DJ Delorie]

Could the #ifdef from b4a5d26d8835's kernel_sigaction.h be backported
instead?  It still wouldn't be a pristine backport (I'm not recommending
backporting *all* of b4a5d26d8835) but at least as branch-specific
'hack' it's closer to what's in master than a riscv-specific header.

Granted, this means a riscv-specific patch *might* affect other targets,
but if that's not an issue, I think something closer to master would be
preferred.