Fix name of wxrc executable when cross-compiling from Unix.
Note that the Makefile hasn't been rebaked after the changes of
f3bd129568 (Append WX_FLAVOUR to the name of wxrc executable,
2020-08-06), so this commit reflects the changes to wxrc.bkl from both
that commit and this one.
Closes https://github.com/wxWidgets/wxWidgets/pull/2041