xtensa: refactor thread related stuff into its own file...

... from xtensa_asm2.c.

Everything has been stuffed inside xtensa_asm2.c where
they are all mangled together. So extract thread related
stuff into its own file.

Note that arch_float_*() may not be thread related but
most other architectures put them into thread.c. So we
also do it here.

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