alpha: add special umount2 handling

Only on Linux alpha __NR_oldumount is defined and a umount not
umount2 syscall, but with two parameter is used.
Add special handling for it and an alias for umount2() users.

There was a discussion about this special handling, but it seems
it was never committed upstream:
http://marc.info/?l=linux-alpha&m=137455037930738&w=2

Runtime tested with qemu-alpha and a statically linked busybox
binary.

libc/sysdeps/linux/common/umount2.c

@@ -10,7 +10,12 @@
 #include <sys/syscall.h>
 
 #if defined __UCLIBC_LINUX_SPECIFIC__ && defined __NR_umount2
-# include <sys/mount.h>
+#include <sys/mount.h>
 _syscall2(int, umount2, const char *, special_file, int, flags)
 libc_hidden_def(umount2)
 #endif
+
+#if defined __UCLIBC_LINUX_SPECIFIC__ && defined __NR_oldumount
+_syscall2(int, umount, const char *, special_file, int, flags)
+strong_alias(umount,umount2)
+#endif