Remove object files when cleaning

Otherwise something like this will fail:
./otp_build setup --enable-m32-build
make clean
./otp_build setup --enable-m64-build
Fix some compiler warnings
Fix broken cpu_sup:nprocs and others on Solaris 64-bit

The correct kstat datatype for nproc & avenrun_* is ui32, not
ulong. Under 64-bit OTP builds garbage was returned because the wrong
datatype was used.

This patch correcs the datatype. It also adds an additional check
in the test case for a slightly less insane return value.