target/arm: Implement v8.4-RCPC

target/arm: Implement v8.4-RCPC

The v8.4-RCPC extension implements some new instructions:
 * LDAPUR, LDAPURB, LDAPURH, LDAPRSB, LDAPRSH, LDAPRSW
 * STLUR, STLURB, STLURH

These are all in a new subgroup of encodings that sits below the
top-level "Loads and Stores" group in the Arm ARM.

The STLUR* instructions have standard store-release semantics; the
LDAPUR* have Load-AcquirePC semantics, but (as with LDAPR*) we choose
to implement them as the slightly stronger Load-Acquire.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200224172846.13053-4-peter.maydell@linaro.org