libc: fix WITHOUT_NIS builds

[emaste]

Summary

A number of xdr_* functions are not built under WITHOUT_NIS, and must
be conditional in the Symbol.map.

Reported by:    matteo
Reported by:    Michael Dexter's Build Option Survey run

[emaste]

Hrm, we don't pass CFLAGS to symver's use of cpp. I'll give that a try.

[imp]

Should do the trick.

[emaste]

I see @peterj also reported this in PR279270 and points out there is already a lib/libc/yp/Symbol.map; moving the symbols there is probably better. But I suspect D45346 is still a good idea to support future cases similar to this.

[emaste]

D45347 to move the symbols instead

[brooks]

Digging in to this, these should certainly just move to lib/libc/yp/Symbol.map. There are even a couple xdr_ symbols there already.

[emaste]

D45347 instead