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)

 --- glibc-2.12.1/sysdeps/unix/sysv/linux/kernel-features.h	2010-07-27 14:34:39.000000000 +0300

 +++ glibc-2.12.1/sysdeps/unix/sysv/linux/kernel-features.h.new	2011-03-10 18:54:37.686795979 +0200

 @@ -437,7 +437,8 @@

     the code.  On PPC they were introduced in 2.6.17-rc1,

     on SH in 2.6.19-rc1.  */

  #if __LINUX_KERNEL_VERSION >= 0x020611 \

 -    && (!defined __sh__ || __LINUX_KERNEL_VERSION >= 0x020613)

 +    && (!defined __sh__ || __LINUX_KERNEL_VERSION >= 0x020613) \

 +    && (!defined __alpha__)

  # define __ASSUME_ATFCTS	1

  #endif