move riscv64 register index constants to signal.h

under _GNU_SOURCE for namespace cleanliness, analogous to other archs.
the original placement in sys/reg.h seems not to have been motivated;
such a header isn't even present on other implementations.

arch/riscv64/bits/reg.h

@@ -1,8 +1,2 @@
 #undef __WORDSIZE
 #define __WORDSIZE 64
-#define REG_PC 0
-#define REG_RA 1
-#define REG_SP 2
-#define REG_TP 4
-#define REG_S0 8
-#define REG_A0 10

arch/riscv64/bits/signal.h

@@ -35,6 +35,15 @@ typedef struct mcontext_t {
 	union __riscv_mc_fp_state __fpregs;
 } mcontext_t;
 
+#if defined(_GNU_SOURCE)
+#define REG_PC 0
+#define REG_RA 1
+#define REG_SP 2
+#define REG_TP 4
+#define REG_S0 8
+#define REG_A0 10
+#endif
+
 #if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
 typedef unsigned long greg_t;
 typedef unsigned long gregset_t[32];