The go runtime generates profiles in profile.proto format
without symbols or mappings. The expectation is that these
can be symbolized by passing the binary name to pprof.

The mechanism pprof uses relies to override the binary relies
on there being a mapping, and previously we moved the creation
of fake mappings to the legacy profile handlers, so profiles
parsed from profile.proto with no mappings can no longer be
symbolized.

Special case this situation to create a fake mapping and associate
all locations to it if there is a command line override but no
mappings.