- make this work on arch's where 'uname -m' != 'uname -p', for example

- make this work on arch's where 'uname -m' != 'uname -p', for example
  the m68k and mipsel ports.  This address PR 9781 filed by Michael Wolfson
  and also fixes recently noted bulk pkgsrc build failures on pmax.

  The os version/machine type specific string is now consistently
  ${OPSYS}-${OS_VERSION}-${MACHINE_ARCH}