ltp: don't use host objcopy

ltp: don't use host objcopy

The kernel/kvm test uses the host objcopy when building a payload, but
the host objcopy might not know how to deal with target binaries:

  CC testcases/kernel/kvm/lib_host.o
  objcopy: Unable to recognise the format of the input file `kvm_svm03-payload.elf'
  make[3]: *** [ltp/testcases/kernel/kvm/Makefile:67: kvm_svm03-payload.o] Error 1

Solve this by using the host-prefixed objcopy binary.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>