s390: Fix undefined behaviour in feenableexcept, fedisableexcept [BZ #30960]

If feenableexcept or fedisableexcept gets excepts=FE_INVALID=0x80
as input, we have a signed left shift: 0x80 << 24 which is not
representable as int and thus is undefined behaviour according to
C standard.

This patch casts excepts as unsigned int before shifting, which is
defined.

For me, the observed undefined behaviour is that the shift is done
with "unsigned"-instructions, which is exactly what we want.
Furthermore, I don't get any exception-flags.

After the fix, the code is using the same instruction sequence as
before.