use namespace-safe __lseek for __stdio_seek instead of direct syscall

this probably saves a few bytes, avoids duplicating the clunky
lseek/_llseek syscall convention in two places, and sets the stage for
fixing broken seeks on x32 and mipsn32.

src/include/unistd.h

@@ -9,5 +9,6 @@ hidden int __dup3(int, int, int);
 hidden int __mkostemps(char *, int, int);
 hidden int __execvpe(const char *, char *const *, char *const *);
 hidden int __aio_close(int);
+hidden off_t __lseek(int, off_t, int);
 
 #endif