Summary:
Add missing calling convention to function pointer,
incorrect `cdecl` calling convention which should be `stdcall`
on x86 was causing the stack to be corrupted. When it tried to
return from the function the return pointer would be invalid.

Test Plan: ./validate

Reviewers: austin, erikd, bgamari, thomie

Reviewed By: bgamari, thomie

Differential Revision: https://phabricator.haskell.org/D1683

GHC Trac Issues: #11234