bf gen: Removed some undefined bit shifts.

The bitfield generator constructs tags as enums. The compiler is free to back
these by any type big enough to cover the enum. The generator emits code that
uses these tag values in mask and shift operations where the target type is
typically a uint32_t. As a result, it can produce code involving undefined
shifts like:

 (seL4_CapData_Badge & 0x1) << 31

This commit casts the tag value to the unsigned target type before masking to
ensure everything is done on an appropriate sized unsigned type.

JIRA: SELFOUR-193