[lit] [Windows] Print exit codes > 255 as hex too

891bb487 made negative exit codes
be printed as hex, which makes it easier to recognize e.g.
0xC0000005 instead of -1073741819. However, current Python versions
(at least the ones I'm using) seem to end up with positive unsigned
return codes, so that again ends up printed as 3221225477.

Print any return code over 255 as a hexadecimal number instead.

Differential Revision: https://reviews.llvm.org/D137771