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.

arch/riscv64/bits/signal.h

@@ -6,12 +6,6 @@
 # define SIGSTKSZ 8192
 #endif
 
-#if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
-typedef unsigned long greg_t;
-typedef unsigned long gregset_t[32];
-typedef union __riscv_mc_fp_state fpregset_t;
-#endif
-
 typedef unsigned long __riscv_mc_gp_state[32];
 
 struct __riscv_mc_f_ext_state {
@@ -41,6 +35,16 @@ typedef struct mcontext_t {
 	union __riscv_mc_fp_state __fpregs;
 } mcontext_t;
 
+#if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
+typedef unsigned long greg_t;
+typedef unsigned long gregset_t[32];
+typedef union __riscv_mc_fp_state fpregset_t;
+struct sigcontext {
+	gregset_t gregs;
+	fpregset_t fpregs;
+};
+#endif
+
 struct sigaltstack {
 	void *ss_sp;
 	int ss_flags;