aarch64.risu: add load exclusive insns for 8/16bit data
aarch64.risu: add load exclusive insns for 8/16bit data

add LDXRB, LDAXRB, LDARB, LDXRH, LDAXRH, LDARH.

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

vivier/risu,stsquad/risu,vivier/risu,stsquad/risu,stsquad/risu