ld: pe: Make archive member file extension comparisons case insensitive when cross...

ld: pe: Make archive member file extension comparisons case insensitive when cross compiling too

On Windows, filename_cmp is case insensitive, but when cross compiling
with libraries that may contain members with uppercase file names, we
should keep those comparisons case insensitive when running the build
tools on other OSes too.

Also make the check for .def consistent with the other ones, fixing
out of bounds reads if file names are shorter than 4 characters.