aarch64.risu: add first memory instruction (STXRB)
aarch64.risu: add first memory instruction (STXRB)

our very first memory instruction. Seems to work for now.
Covering the whole range of addressing modes will be hard with
the existing risu ARM assumptions.

Signed-off-by: Claudio Fontana <5fe97b97070bc75ff3c7a6b130e2dcc62cbb9b58@linaro.org>

hw-claudio/risu-aarch64,hw-claudio/risu-aarch64