build: prepend OBJ_DIR to linker script

build: prepend OBJ_DIR to linker script

After 5623e2d2 ("x86: use unified linker script") the linker script for
x86 build is generated. But the special rule to generate linker script
doesn't have OBJ_DIR prepended so in parallel build the same file is
written multiple times. This is racy and would cause parallel build to
fail.

Fix this by prepending OBJ_DIR to the path of linker script. Change
other variables where necessary.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>