thunk: Rename args and fields in host-target bitmask conversion code

thunk: Rename args and fields in host-target bitmask conversion code

The target_to_host_bitmask() and host_to_target_bitmask() functions
and the associated struct bitmask_transtbl are completely generic,
but for historical reasons the target related fields and parameters
are named 'x86' and the host related fields are named 'alpha'.
Rename them to 'target' and 'host'.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>