COFF: When generating code for LTO, use static reloc model on 32-bit x86.

Fixes PR34306.

This is because it usually results in more compact code, and because
there are also known code generation bugs when using the PIC model
(see bug).

Based on a patch by Carlo Kok.

Differential Revision: https://reviews.llvm.org/D38769

llvm-svn: 315400