sstate-diff-machines.sh: Replace MACHINE_ARCH only at the beginning and separated with dash

* I had some components where the MACHINE name was also included in PV of allarch recipe
  which was tripping the script into saying that they have different signatures (because
  for one MACHINE was the string in PV replaced with 'MACHINE' and not for other MACHINEs

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>