target/nios2: Implement Misaligned destination exception

target/nios2: Implement Misaligned destination exception

Indirect branches, plus eret and bret optionally raise
an exception when branching to a misaligned address.
The exception is required when an mmu is enabled, but
enable it always because the fallback behaviour is not
documented (though presumably it discards low bits).

For the purposes of the linux-user cpu loop, if EXCP_UNALIGN
(misaligned data) were to arrive, it would be treated the
same as EXCP_UNALIGND (misaligned destination).  See the
!defined(CONFIG_NIOS2_ALIGNMENT_TRAP) block in kernel/traps.c.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220421151735.31996-53-richard.henderson@linaro.org>