avoid use of readv syscall in __stdio_read backend when not needed

formally, calling readv with a zero-length first iov component should
behave identically to calling read on just the second component, but
presence of a zero-length iov component has triggered bugs in some
kernels and performs significantly worse than a simple read on some
file types.

src/stdio/__stdio_read.c

@@ -9,7 +9,8 @@ size_t __stdio_read(FILE *f, unsigned char *buf, size_t len)
 	};
 	ssize_t cnt;
 
-	cnt = syscall(SYS_readv, f->fd, iov, 2);
+	cnt = iov[0].iov_len ? syscall(SYS_readv, f->fd, iov, 2)
+		: syscall(SYS_read, f->fd, iov[1].iov_base, iov[1].iov_len);
 	if (cnt <= 0) {
 		f->flags |= cnt ? F_ERR : F_EOF;
 		return 0;