"hardfloat" Test in qtjsbackend Uses "readelf" Directly

The pro file qtjsbackend/config.tests/hardfloat/hardfloat.pro uses readelf directly, rather than the one specified in a cross toolchain.

I think there are two issues here:
1. The test should fail if it cannot find readelf as opposed to assuming no hardfloat support when readelf is missing.

2. The test should use readelf provided by a cross toolchain.