llvm-lib: Disallow mixing object files with different machine types

lib.exe doesn't allow creating .lib files with object files that have
differing machine types. Update llvm-lib to match.

The motivation is to make it possible to infer the machine type of a
.lib file in lld, so that it can warn when e.g. a 32-bit .lib file is
passed to a 64-bit link (PR38965).

Fixes PR38782.

Differential Revision: https://reviews.llvm.org/D62913

llvm-svn: 362798