Change the data type of libvex_Backend::emit.

Change the data type of libvex_Backend::emit.

Both the s390 and arm insn emitters need to know the host's hardware
capabilities. Today, these are provided by means of global variables
s390_host_hwcaps and arm_hwcaps. To eliminate that kludge the emit
function is changed. Instead of passing VexEndness we now pass a pointer
to VexArchInfo which provides both the endianess and hardware capabilities.

Those global variables will be removed in a followup patch.