Give up earlier on MIPS64 and don't tell users it is supported

Chromium doesn't currently build on MIPS64, so we shouldn't tell people
it might work.

Task-number: QTBUG-62655
Change-Id: I56e798cae6914cc8087f1a6a51f07383e81f86c1
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>