sysrand: fix syscall signature [backport] (#19982)

sysrand: fix syscall signature

`syscall` is a `C` varags function

lib/std/sysrand.nim

@@ -170,9 +170,8 @@ elif defined(linux) and not defined(nimNoGetRandom) and not defined(emscripten):
   const syscallHeader = """#include <unistd.h>
 #include <sys/syscall.h>"""
 
-  proc syscall(
-    n: clong, buf: pointer, bufLen: cint, flags: cuint
-  ): clong {.importc: "syscall", header: syscallHeader.}
+  proc syscall(n: clong): clong {.
+      importc: "syscall", varargs, header: syscallHeader.}
     #  When reading from the urandom source (GRND_RANDOM is not set),
     #  getrandom() will block until the entropy pool has been
     #  initialized (unless the GRND_NONBLOCK flag was specified).  If a