powerpc32: fix stack allocation code generation

When ghc was built for powerpc32 built failed as:

It's a fallout of commit 3f46cffc
("PPC NCG: Refactor stack allocation code") where
word size used to be
    II32/II64
and changed to
    II8/panic "no width for given number of bytes"
    widthFromBytes ((platformWordSize platform) `quot` 8)

The change restores initial behaviour by removing extra division.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

compiler/nativeGen/PPC/Instr.hs

@@ -98,7 +98,7 @@ ppc_mkStackAllocInstr' platform amount
     , STU fmt r0 (AddrRegReg sp tmp)
     ]
   where
-    fmt = intFormat $ widthFromBytes ((platformWordSize platform) `quot` 8)
+    fmt = intFormat $ widthFromBytes (platformWordSize platform)
     zero = ImmInt 0
     tmp = tmpReg platform
     immAmount = ImmInt amount