cmd/viewcore: display the origin of core

This change adds the origin of the analyzed core in the welcome
message of interactive mode. The information is from NT_PRPSINFO
(currently assuming linux/amd64 only). This info allows users to
verify they are looking into the correct core file.
This is similar to what gdb does.

Change-Id: Ib626c9007a588e34cf633908dedde591e6f04e3c
Reviewed-on: https://go-review.googlesource.com/121015
Reviewed-by: Heschi Kreinick <heschi@google.com>