Fix ARM build  on KVM

-> Use system ld binary in case of ARM build

->Unset  sysroot variable build is done on KVM

Help to fix bug XWALK-1673

Change-Id: I185f1891faca90f7ecfc3a0c62edcaa838d4882e
Signed-off-by: Baptiste DURAND <baptiste.durand@open.eurogiciel.org>