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)

 diff -durN glibc-2.7.orig/Makerules glibc-2.7/Makerules

 --- glibc-2.7.orig/Makerules	2007-08-26 04:18:03.000000000 +0200

 +++ glibc-2.7/Makerules	2008-07-28 15:17:27.000000000 +0200

 @@ -1203,6 +1203,9 @@

  # The include magic above causes those files to use this variable for flags.

  CPPFLAGS-nonlib = -DNOT_IN_libc=1

 +ifneq ($(OBJDUMP_FOR_HOST),)

 +OBJDUMP = $(OBJDUMP_FOR_HOST)

 +endif

  ifeq ($(versioning),yes)

  # Generate normalized lists of symbols, versions, and data sizes.