Fix compilation of `integer-gmp2` with `-O0`

At optimization level `-O0` `wordToInteger` wasn't inlined, and this caused a
`__integer` literal to turn up in final Core, which would trigger the GHC panic

  ghc-stage1: panic! (the 'impossible' happened)
    (GHC version 7.9.20141114 for x86_64-unknown-linux):
	Can't use Integer in integer-*

By forcing inlining via `inline` this is avoided.
This should hopefully address #9800.