target-arm: Fix raw read and write functions on AArch64 registers

target-arm: Fix raw read and write functions on AArch64 registers

The raw read and write functions were using the ARM_CP_64BIT flag in
ri->type to determine whether to treat the register's state field as
uint32_t or uint64_t; however AArch64 register info structs don't use
that flag. Abstract out the "how big is the field?" test into a
function and fix it to work for AArch64 registers. For this to work
we must ensure that the reginfo structs put into the hashtable have
the correct state field for their use, not the placeholder STATE_BOTH.

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