PPC NCG: Fix pretty printing of st[wd]ux instr.
PPC NCG: Fix pretty printing of st[wd]ux instr.

Printing STU was mixed up. The tab character must appear
after the 'x'.

Test Plan: validate on powerpc

Reviewers: bgamari, austin, erikd

Reviewed By: austin, erikd

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2214

GHC Trac Issues: #12054

(cherry picked from commit 2dbdc79bb9e2760394ebfe630908813b630146c7)

GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc