Correct incorrect free in PE linker
The big-obj support (D3523) had introduced an early free on
the info structure. Because the pointer is not NULL'd
and the default of all the utility functions was to the
standard object format, it all kept working.
The one big-obj test that exists was subjected to a timing issue.
usually the test ran quickly enough that the allocator hasn't
had time to reclaim the memory yet, so it still passed.
This corrects it. Also as it so happens, static LLVM libraries
from mingw-w64 are compiled using big-obj.
Test Plan: ./validate
Reviewers: austin, bgamari, erikd, simonmar
Reviewed By: bgamari
Subscribers: rwbarton, thomie
Differential Revision: https://phabricator.haskell.org/D3862