Remove the gating in moz.configure that prevents the arm64 simulator from
being compiled on an arm64 system.

Avoid using the fjcvt instruction when running on arm64 hardware, since
the simulator does not appear to support it while cpu detection does
support it.  (The alternative would be to implement the instruction
on the simulator.)

Differential Revision: https://phabricator.services.mozilla.com/D147199