S390: Implement libc_fe* macros.

This patch provides the s390 specific implementation for
libc_feholdexcept, libc_fesetround, libc_feholdexcept_setround,
libc_fetestexcept, libc_fesetenv, libc_feupdateenv_test,
libc_feupdateenv, libc_feholdsetround_ctx, libc_feresetround_ctx,
libc_feholdsetround_noex_ctx and libc_feresetround_noex_ctx.