s390x: avoid variable-length arrays in struct for asm operands

s390x: avoid variable-length arrays in struct for asm operands

... since it is not required and clang does not support that gcc
extension. Instead, use a variable-length array directly for these
operands.

Note that, while the actual inline assembly code does not directly use
these memory operands, they serve to inform the compiler that it cannot
reorder reads or writes to/from the input and output data across the
inline asm statements.

Signed-off-by: Marius Hillenbrand <mhillen@linux.ibm.com>