[mkbundle] Encode directory separator character on Windows (#4493)

[mkbundle] Encode directory separator character on Windows (#4493)

When storing satellite assemblies, mkbundle prefixes their names with a
directory name derived from the language/locale of the assembly. It uses the
platform's default directory separator character which on Windows defaults to
'\' and that causes problems when building such a bundle on Windows since \ is
an escape sequence prefix inside strings and not escaping it with another \
leads to gcc errors when building the genrated source.

This commit fixes the problem by quoting the directory separator character
properly on Windows.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=52845