Stop saving floating point registers in __morestack

[brson]

They require 128 bytes of stack and account for a big chunk of the work. We started saving them because of #1388 but we should be able to acquire a new stack segment without using floating point registers. GCC doesn't save them.

[luqmana]

I couldn't reproduce the weirdness in the test case from #1388 when I got rid of the saves.