s390: add assembler macros for CPU alternatives

Add a header with macros usable in assembler files to emit alternative
code sequences. It works analog to the alternatives for inline assmeblies
in C files, with the same restrictions and capabilities.
The syntax is

     ALTERNATIVE "<default instructions sequence>", \
		 "<alternative instructions sequence>", \
		 "<features-bit>"
and

     ALTERNATIVE_2 "<default instructions sequence>", \
		   "<alternative instructions sqeuence #1>", \
		   "<feature-bit #1>",
		   "<alternative instructions sqeuence #2>", \
		   "<feature-bit #2>"

Reviewed-by: Vasily Gorbik <gor@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>