Fix the RPi3 spec a bit

So that it actually compiles with gcc-linaro-arm-linux-gnueabihf-raspbian.

Remove also flags a device spec should not set (like -std and -O).

Change-Id: Ib7a3bc298e60715410d5c00fbc22199ab4711fa7
Reviewed-by: Andy Nichols <andy.nichols@qt.io>