rt: Convert timezone to utf-8 on Windows

Previously #9418 fixed utf-8 assertion issue by wcsftime,
but the function didn't work as expected: it follows the locale
set by setlocale(), not the system code page.
This patch fixes it by manual multibyte-to-unicode conversion.
extra::time: Fix test on Windows

Closes #10307
auto merge of #10281 : klutzy/rust/rt-timezone, r=alexcrichton

Previously #9418 fixed utf-8 assertion issue by wcsftime,
but the function didn't work as expected: it follows the locale
set by setlocale(), not the system code page.
This patch fixes it by manual multibyte-to-unicode conversion.