s390/vx: add support functions for in-kernel FPU use

Introduce the kernel_fpu_begin() and kernel_fpu_end() function
to enclose any in-kernel use of FPU instructions and registers.
In enclosed sections, you can perform floating-point or vector
(SIMD) computations.  The functions take care of saving and
restoring FPU register contents and controls.

For usage details, see the guidelines in arch/s390/include/asm/fpu/api.h

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>