powerpc/process: Remove useless #ifdef CONFIG_PPC_FPU

Add a stub for __giveup_fpu() when CONFIG_PPC_FPU is
not selected, as done for CONFIG_SPE and CONFIG_ALTIVEC.

This allows to remove some #ifdef CONFIG_PPC_FPU.

Also change one to IS_ENABLED().

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/69c8b7954ceeccc6b849e52e1fa41b3a0f10f6c1.1597643221.git.christophe.leroy@csgroup.eu