Replaced Linux syscall macros by libc macros.

User-space programs should use the SYS_*form, as documented in
syscall(2).  That also adds compatibility to non-Linux systems.

auto/isolation

@@ -21,7 +21,7 @@ nxt_feature_test="#include <sys/wait.h>
                   #include <sys/syscall.h>
 
                   int main() {
-                      return __NR_clone | SIGCHLD;
+                      return SYS_clone | SIGCHLD;
                   }"
 . auto/feature
 
@@ -68,7 +68,7 @@ nxt_feature_libs=
 nxt_feature_test="#include <sys/syscall.h>
 
                   int main() {
-                      return __NR_pivot_root;
+                      return SYS_pivot_root;
                   }"
 . auto/feature