s390/vx: add vector instruction support for older binutils versions

Older binutils versions do not include support for the vector instruction
formats.  Add assembler macros for vector instruction mnemonics to easily
encode and generate vector instructions.

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