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>

Original patch from gentoo: gentoo/src/patchsets/gcc/4.3.1/gentoo/51_all_gcc-3.4-libiberty-pic.patch

diff -durN gcc-4.3.1.orig/libiberty/Makefile.in gcc-4.3.1/libiberty/Makefile.in

--- gcc-4.3.1.orig/libiberty/Makefile.in	2007-07-25 08:26:45.000000000 +0200

+++ gcc-4.3.1/libiberty/Makefile.in	2008-06-10 14:58:02.000000000 +0200

@@ -225,6 +225,7 @@

 	  $(AR) $(AR_FLAGS) $(TARGETLIB) \

 	    $(REQUIRED_OFILES) $(EXTRA_OFILES) $(LIBOBJS); \

 	  $(RANLIB) $(TARGETLIB); \

+	  cp $(TARGETLIB) ../ ; \

 	  cd ..; \

 	else true; fi