Stop setting WANT_ROUNDING=0.

Remove this based on the explanation on
https://github.com/ARM-software/optimized-routines/issues/23.

Also document WANT_ERRNO=0 (which does make sense for us) based on
https://github.com/ARM-software/optimized-routines/issues/16#issuecomment-572009659.

Test: manually ran all the tests with a clang from the future
Change-Id: I37393633416189854f349606c15d67b51cf0f813