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>

 diff -ru gdb-6.8.orig/bfd/sysdep.h gdb-6.8/bfd/sysdep.h

 --- gdb-6.8.orig/bfd/sysdep.h	2007-07-03 16:26:42.000000000 +0200

 +++ gdb-6.8/bfd/sysdep.h	2011-08-21 13:33:47.000000000 +0200

 @@ -131,7 +131,7 @@

  extern PTR realloc ();

  #endif

 -#if !HAVE_DECL_STPCPY

 +#if !HAVE_DECL_STPCPY && !defined(stpcpy)

  extern char *stpcpy (char *__dest, const char *__src);

  #endif