Windows needs the current codepage instead of utf8 sometimes

Llvm-mc (and tools that use Path.inc on Windows) assume that strings are utf-8 
encoded, however, this is not always the case. On Windows the default codepage 
is not utf-8, so most of the time the strings are not utf-8 encoded.

The lld test 'format-binary-non-ascii' uses llvm-mc with a file with non-ascii 
characters in the name which is how this bug was found. The test fails when run 
using Python 3 because it uses properly encoded unicode strings (Python 2 actually 
ends up using a byte string which is not utf-8 encoded, so the test passes, but 
that's separate issue). 

Patch by Stella Stamenova!

llvm-svn: 329468