[SystemZ][z/OS][libcxx]: fix libcxx test cases related to codecvt class UTF8

This PR to fix a few test cases related to class https://en.cppreference.com/w/cpp/locale/codecvt , as mentioned in document, class is converting UTF16 and UTF8 or UTF32 and UTF8, character type is deprecated in c++20 and it needs explicitly specify it is UTF8 string literal. Current test cases assume 1 byte character is ASCII or Unicode character which is not true on z/OS platform. UTF8/16/32 information can be found in https://naveenr.net/unicode-character-set-and-utf-8-utf-16-utf-32-encoding/ and EBCDIC and ASCII character value can be found in http://www.simotime.com/asc2ebc1.htm

Differential Revision: https://reviews.llvm.org/D106153