checkasm: improve Linux perf error message

Report the failing system call name, as is convention, rather than just
a rather unhelpful "syscall".

tests/checkasm/checkasm.c

@@ -667,7 +667,7 @@ static int bench_init_linux(void)
 
     state.sysfd = syscall(__NR_perf_event_open, &attr, 0, -1, -1, 0);
     if (state.sysfd == -1) {
-        perror("syscall");
+        perror("perf_event_open");
         return -1;
     }
     return 0;