avoid type aliasing

avoid type aliasing

For -O2 gcc activates -fstrict-aliasing. As a result the compiler is
allowed to assume that pt[1]=px[1]/3+B1 does not affect t.

Therefore the result of _fast_cbrt() was always 0.
Using a union for casting avoids this issue.

For more details see:
http://labs.qt.nokia.com/2011/06/10/type-punning-and-strict-aliasing

Also the updated code respect endianness.

Change-Id: Id4bed16efac52e494e7357dc2f23f94e8c525df1
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>