Xtensa ISA got specification for 64-bit floating point registers and
opcodes, see ISA, 4.3.11 "Floating point coprocessor option".

Add 64-bit FP registers.

Although 64-bit floating point is currently not supported by xtensa
translator, these registers need to be reported to gdb with proper size,
otherwise it wouldn't find other registers.
Signed-off-by: NMax Filippov <jcmvbkbc@gmail.com>