cleaning up syscalls in preparation for x86_64 port

- hide all the legacy xxxxxx32 name cruft in syscall.h so the actual
source files can be clean and uniform across all archs.

- cleanup llseek/lseek and mmap2/mmap handling for 32/64 bit systems

- alternate implementation for nice if the target lacks nice syscall

src/stdio/__stdio_seek.c

@@ -7,7 +7,13 @@ static off_t retneg1(FILE *f, off_t off, int whence)
 
 off_t __stdio_seek(FILE *f, off_t off, int whence)
 {
-	off_t ret = __syscall_lseek(f->fd, off, whence);
+	off_t ret;
+#ifdef __NR__llseek
+	if (syscall5(__NR__llseek, f->fd, off>>32, off, (long)&ret, whence)<0)
+		ret = -1;
+#else
+	ret = syscall3(__NR_lseek, f->fd, off, whence);
+#endif
 	/* Detect unseekable files and optimize future failures out */
 	if (ret < 0 && off == 0 && whence == SEEK_CUR)
 		f->seek = retneg1;