S390: Use libc_fe* macros in fe* functions.

This patch updates the s390 specific functions fegetround,
fesetround, feholdexcept, fesetenv, feupdateenv, fegetexceptflag,
fetestexcept, fesetexceptflag, fetestexceptflag.
Now those functions are using the libc_fe* macros if possible.

Furthermore fegetexceptflag is now returning the exception from
dxc field shifted to the usual exception-flags.
Thus a special fetestexceptflag implementation is not needed anymore.