fdo#77979: Make rtl_TextEncodingToWinCharsetRTF work for non-Unicode

"LC_ALL=C make CppunitTest_sw_rtfexport" was still failing after
e47a02b1524061143d8e77a54eb95c77f2e6dae2 "fdo#77979: sw: RTF export: write non-
ASCII font names encoded," so for each given eTextEncoding determine a Windows
charset that can at least encode the font's name, and not only for Unicode-
related eTextEncodings.

Change-Id: If547566bb0cffc60411d8f667d76749a904f7a3f