Clear up strxfrm() in UTF-8 with locale on Windows

The attached patch clears up the usage of strxfrm() on Windows. If the
server encoding is UTF-8 and the locale is not C, we should use wcsxfrm()
instead of strxfrm() because UTF-8 locale are not supported on Windows.
We've already have a special version of strcoll() for Windows, but the
usage of strxfrm() was still broken.

When we are caught up in the bug, we see the next error message.
| ERROR: invalid memory alloc request size 2147483648
If the server is wrong configured between the server encoding and the
locale, strxfrm() could be failed and return values like INT_MAX or
(size_t)-1. We've passed the result+1 straight to palloc(), so the server
tried to allocale more than 1GB of memory and gave up.

Regards,
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ITAGAKI Takahiro
NTT Open Source Software Center