reintroduce riscv64 struct sigcontext

reintroduce riscv64 struct sigcontext

commit ab3eb89a8b83353cdaab12ed017a67a7730f90e9 removed it as part of
correcting the mcontext_t definition, but there is still code using
struct sigcontext and expecting the member names present in it, most
notably libgcc_eh. almost all such usage is incorrect, but bring back
struct sigcontext at least for now so as not to introduce regressions.