target/xtensa: support copying registers up to 64 bits wide

target/xtensa: support copying registers up to 64 bits wide

FLIX dependency breaking code assumes that all registers are 32 bit
wide. This may not always be correct.
Extract actual register width from the associated register file and use
it to create temporaries of correct width and generate correct data
movement instructions.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>