xtensa: move FPU_REG_SAVE out of ODD_REG_SAVE

Syscall entrance makes use of ODD_REG_SAVE but it does not
really need to save FPU registers as it is technically
the same thread and same context. So extract call to
FPU_REG_SAVE to interrupt handling code.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>