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 b4a5d26d8835 ("linux: Consolidate sigaction implementation").
b88e5137e6