linux: Remove INTERNAL_SYSCALL_DECL

With all Linux ABIs using the expected Linux kABI to indicate
syscalls errors, the INTERNAL_SYSCALL_DECL is an empty declaration
on all ports.

This patch removes the 'err' argument on INTERNAL_SYSCALL* macro
and remove the INTERNAL_SYSCALL_DECL usage.

Checked with a build against all affected ABIs.

sysdeps/unix/sysv/linux/dl-writev.h

@@ -33,6 +33,5 @@
 static inline void
 _dl_writev (int fd, const struct iovec *iov, size_t niov)
 {
-  INTERNAL_SYSCALL_DECL (err);
-  INTERNAL_SYSCALL (writev, err, 3, fd, iov, niov);
+  INTERNAL_SYSCALL_CALL (writev, fd, iov, niov);
 }