Implement the SET* instructions which collectively implement a
"memset" operation.  These come in a set of three, eg SETP
(prologue), SETM (main), SETE (epilogue), and each of those has
different flavours to indicate whether memory accesses should be
unpriv or non-temporal.

This commit does not include the "memset with tag setting"
SETG* instructions.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20230912140434.1333369-8-peter.maydell@linaro.org