scripts: fix sysroot prefix dir

The sysroot prefix dir was broken in #4960f5d9f829 due to a mishap
when making the out-of-sysroot lib/ symlink: the './' was mistakenly
changed into a single '.' .

Although Jonathan suggested restoring the missing '/' to restore it to
normal operation, I prefered using an explicit pushd/popd to be extra
sure of the symlink location and target, along with a fix in the sysroot
relative directory calculation.

Reported-by: Jonathan Grundon <JGrundon@xos.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
(transplanted from e5fc5c9ea78af28d05244ba09cf718cf75470903)

alpha does not have a __NR_creat

http://bugs.gentoo.org/227275

http://sourceware.org/bugzilla/show_bug.cgi?id=6650

diff -durN glibc-2.12.1.orig/sysdeps/unix/sysv/linux/wordsize-64/creat64.c glibc-2.12.1/sysdeps/unix/sysv/linux/wordsize-64/creat64.c

--- glibc-2.12.1.orig/sysdeps/unix/sysv/linux/wordsize-64/creat64.c	2007-11-10 20:34:26.000000000 +0100

+++ glibc-2.12.1/sysdeps/unix/sysv/linux/wordsize-64/creat64.c	2009-11-13 00:50:51.000000000 +0100

@@ -1 +1,5 @@

 /* Defined as alias for the syscall.  */

+#include <sysdep.h>

+#ifndef __NR_creat

+#include "../../../../../io/creat64.c"

+#endif