jakzale on #ghc reported a build failure
when ported GHC on a new target.

The code 'pprHexVal (2^32) W32' emits '0xU'
which is invalid C.

I've introduced bug in
    43f1b2ec
when added literal truncation. That truncation
is a new source of zeros.

Signed-off-by: Sergei Trofimovich <siarheit@google.com>

Test Plan: added test and tested on UNREG ghc

Reviewers: austin

Reviewed By: austin

Subscribers: thomie, bgamari

Differential Revision: https://phabricator.haskell.org/D987

GHC Trac Issues: #10518

(cherry picked from commit a5084557)