[S390] __FD_foo definitions.

Make the definitions of __FD_SET, __FD_CLR and __FD_ISSET independent
from asm/bitops.h and remove the macro magic that tests for __GLIBC__.
Use simple C inline functions instead of set_bit, clear_bit and test_bit.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>