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 -durN gcc-4.4.0.orig/libgomp/configure gcc-4.4.0/libgomp/configure

--- gcc-4.4.0.orig/libgomp/configure	2009-04-21 11:08:08.000000000 +0200

+++ gcc-4.4.0/libgomp/configure	2009-05-27 21:38:19.000000000 +0200

@@ -3334,7 +3334,7 @@

 # Add -Wall -Werror if we are using GCC.

 if test "x$GCC" = "xyes"; then

-  XCFLAGS="$XCFLAGS -Wall -Werror"

+  XCFLAGS="$XCFLAGS -Wall"

 fi

 # Find other programs we need.