[MXNET-260]remove use_fast_math

[yajiedesign]

Description

remove use_fast_math because it causes some operations to appear Nan

https://issues.apache.org/jira/browse/MXNET-260

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [MXNET-$JIRA_ID], where $JIRA_ID refers to the relevant JIRA issue created (except PRs with tiny changes)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage:

Changes

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here

[sxjscience]

This solves #9572 . Also, I find that (-1)**2 and (-1)**3 are not tested in the code. We'd better test these cases here (https://github.com/apache/incubator-mxnet/blob/master/tests/python/unittest/test_operator.py#L431-L438).

[piiswrong]

Also need to turn it off for Makefile?

[szha]

Makefile doesn't have that flag for nvcc compilation lines. It doesn't seem like the cmake development includes the consistency of compilation flags as part of the requirement.