rtld: unconditionally generate map file during build

[kib]

Summary

It is needed at least to ensure that undesirable code is not linked into
rtld from libsys/libc, and adding the map file option each time is not
productive.

[emaste]

No objection but I would suggest changing the commit message slightly -- right now it seems to imply that the mere presence of the map file prevents undesired libc/libsys code into rtld, rather than just always having the map file available for the developer to inspect. Or just a comment before the LDFLAGS entry, something like

# Always produce the map file so that may be inspected to confirm undesired
# code is not linked from libsys/libc.

[kib]

Incorporate suggested comment.
Automate some checks I intended to have.

[emaste]

Thanks, this makes it more clear why we do this.

You may want to redirect error messages to stderr.