Move pointer size detection entirely to qprocessordetection.h

This commit removes the legacy ptrsize check, which was deficient
because it did not work for multiarch systems (when we supported fat
OS X binaries) and did not work for bootstrap builds because the size
might be different when cross-compiling.

Instead, let's rely on the predefined preprocessor macros to detect
correctly. As a nice side-effect, this fixes 64-bit Android builds
cross-compiled from Windows.

Task-number: QTBUG-48932
Change-Id: I1d0f78915b5942aab07cffff140f9a52b9342f23
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>