Fix issue #20821

[lshariprasad]

TODO: Fix C string encoding in mypyc/codegen/cstring.py

Issue

The current implementation uses octal escape sequences (\XXX) but the tests expect hex escape sequences (\xXX).

This are the main changes maded :

1. Understand the expected behavior from tests in test_emitfunc.py
2. Update CHAR_MAP to use hex escapes instead of octal escapes
3. Keep simple escape sequences for special chars (\n, \r, \t, etc.)
4. Update the docstring to reflect correct format (\xXX instead of \oXXX)