s390/vx: add vx-insn.h wrapper include file

s390/vx: add vx-insn.h wrapper include file

The vector instruction macros can also be used in inline assemblies. For
this the magic

asm(".include \"asm/vx-insn.h\"\n");

must be added to C files in order to avoid that the pre-processor
eliminates the __ASSEMBLY__ guarded macros. This however comes with the
problem that changes to asm/vx-insn.h do not cause a recompile of C files
which have only this magic statement instead of a proper include statement.
This can be observed with the arch/s390/kernel/fpu.c file.

In order to fix this problem and also to avoid that the include must
be specified twice, add a wrapper include header file which will do
all necessary steps.

This way only the vx-insn.h header file needs to be included and changes to
the new vx-insn-asm.h header file cause a recompile of all dependent files
like it should.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>