Tidy up the no-external-assembler diag

Diags aren't usually in the first person, and 'windows' isn't the correct
product spelling to use in prose. Sidestep issues completely by making this
error message platform-neutral.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195422 91177308-0d34-0410-b5e6-96231b3b80d8