target-xtensa: add 64-bit floating point registers 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: Max Filippov <jcmvbkbc@gmail.com>