stdlib: change the console to UTF-8 on start
This adjusts the Windows console to switch the codepage to UTF-8. This
is important as the default codepage (CP437) does not allow for UTF-8
output, but expects ASCII. However, strings in Swift are assumed to be
UTF-8, which means that there is now a conversion mismatch.
Because the console mode persists beyond the duration of the application
as it is state local to the console and not the C runtime, we should
restore the state of the console before termination. We do this by
registering a termination handler via `atexit`. This means that an
abnormal termination (e.g. via `fatalError`) will irrevocably alter the
state of the console (interestingly enough, `chcp` will still report the
original console codepage even though the console will internally be set
to UTF-8).
Fixes: SR-13807
(cherry picked from commit 9847cab8d3e5b74101a54bd3871f5002ffaf688c)
