csh: Remove gethost dependency on tc.const.h

[emaste]

Summary

gethost is a build tool built in stage 2.3, but it had a dependency on
tc.const.h, which requires target headers installed in stage 4.1.  The
build falls back to the host's headers if the target headers don't yet
exist, which may result in a build failure.

As gethost.c doesn't actually require the definitions in tc.const.h add
a hack to skip the include of tc.const.h.

PR: 283273
Fixes: e754e5f36195 ("Upgrade to 6.10")

[emaste]

Add a comment explaining the csh build hack