target-arm: Make SETEND respect bswap_code (BE8) setting

target-arm: Make SETEND respect bswap_code (BE8) setting

Make the SETEND instruction respect the setting of bswap_code,
so that in BE8 mode we UNDEF for attempts to switch into
little-endian mode and nop for attempts to stay in big-endian
mode. (This is the inverse of the existing handling of SETEND
in the more common little-endian setup, which we use since
we don't implement the architecturally-mandated dynamic
endianness switching.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>