What is the meaning for the lines "if you can bear to hear the truth you've spoken...........And stoop and build 'em up with worn-out tools"?

One of the major themes of this poem is that the superior
person, anyone who is fit to be called a "man," is someone who will do their best at all
times.  They will work hard and never give up, even when they are surrounded by lesser
people who do not appreciate their work and who may even destroy it.  This is the
meaning of the lines you cite.

Kipling is saying that you
may do and say great things and other people may distort or destroy them.  They might do
these things either because they are fools or because they are bad people.  Either way,
the superior "man" just takes that in stride.  The superior man sees his work or his
words destroyed and just gets back to work.

So, these lines
are saying that you never give up and you never allow other people to affect what you
do.  If you know you are doing right, you just keep doing it, even if other people are
destroying your work and even if you are worn out and tired from your
efforts.