cmake: Pass the -m32 flag to modules if LLVM_BUILD_32_BITS is enabled

This was previously only done for executables and shared libraries, but not
for modules. As modules are essentially shared libraries (that need to be
dlopened explicitly), threating them the same as shared libraries seems
reasonable. This fixes the LLVM_BUILD_32_BITS build of Polly.

Contributed by: Ondra Hosek  <ondra.hosek@gmail.com>

llvm-svn: 158195