In a call to a fixed function the TOC does not need to be saved.
The linker handles TOC saving.

Refactor TOC handling by folding the two functions toc_before and
toc_after into the code generating the call sequence. This saves
repeating the case distinction in those two functions.

Test Plan: validate on PowerPC 32-bit Linux and AIX

Reviewers: hvr, simonmar, austin, erikd, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2328