[flang] Fix ARM/POWER test failure (folding20.f90)

Recent code for folding MINVAL() didn't allow for architectures
whose C/C++ char type is unsigned, so the value of the maximum
Fortran character was incorrect.  This was caught by the
folding20.f90 test.  The fix is to avoid numeric_limits<> and
use hard values for max signed integers of various character kinds.

Pushing into llvm-project/main to restore ARM/POWER buildbots.