Hide unknown registers, which probably don't exist

Change-Id: Iffa8fa5ff4b0a01abd30fa302b7087e2011337bf

src/target/riscv/riscv-013.c

@@ -1249,6 +1249,12 @@ static int init_registers(struct target *target)
 				r->name = csr_info[csr_info_index].name;
 			} else {
 				sprintf(reg_name, "csr%d", csr_number);
+				// Assume unnamed registers don't exist, unless we have some
+				// configuration that tells us otherwise. That's important
+				// because eg. Eclipse crashes if a target has too many
+				// registers, and apparently has no way of only showing a
+				// subset of registers in any case.
+				r->exist = false;
 			}
 
 			switch (csr_number) {

src/target/target.c

@@ -2742,6 +2742,7 @@ COMMAND_HANDLER(handle_reg_command)
 					i < cache->num_regs;
 					i++, reg++, count++) {
 				/* only print cached values if they are valid */
+				if (reg->exist) {
 					if (reg->valid) {
 						value = buf_to_str(reg->value,
 								reg->size, 16);
@@ -2759,6 +2760,7 @@ COMMAND_HANDLER(handle_reg_command)
 								reg->size) ;
 					}
 				}
+			}
 			cache = cache->next;
 		}