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 -dur strace-4.5.16.orig/syscall.c strace-4.5.16/syscall.c

 --- strace-4.5.16.orig/syscall.c	2006-12-21 23:13:33.000000000 +0100

 +++ strace-4.5.16/syscall.c	2007-07-14 19:21:44.000000000 +0200

 @@ -1045,6 +1045,15 @@

  		/*

  		 * Note: we only deal with only 32-bit CPUs here.

  		 */

 +

 +		if (!(tcp->flags & TCB_INSYSCALL) &&

 +		    (tcp->flags & TCB_WAITEXECVE)) {

 +			/* caught a fake syscall from the execve's exit */

 +			tcp->flags &= ~TCB_WAITEXECVE;

 +			return 0;

 +		}

 +

 +

  		if (regs.ARM_cpsr & 0x20) {

  			/*

  			 * Get the Thumb-mode system call number