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

The RISC-V kernel doesn't define SA_RESTORER, hence the kernel version
of struct sigaction doesn't have the sa_restorer field. The default
kernel_sigaction.h therefore can't be used.

This patch adds a RISC-V specific version of kernel_sigaction.h to fix
the issue. This fixes for example the libnih testsuite.

Note that this patch is not needed in master as the bug has been fixed
by commit b4a5d26d ("linux: Consolidate sigaction implementation").