It's okay to preempt at ordinary function calls because
compilers arrange that there are no live registers to save
on entry to the function call.

The software floating point routines are function calls
masquerading as individual machine instructions. They are
expected to keep all the registers intact. In particular,
they are expected not to clobber all the floating point
registers.

The floating point registers are kept per-M, because they
are not live at non-preemptive goroutine scheduling events,
and so keeping them per-M reduces the number of 132-byte
register blocks we are keeping in memory.

Because they are per-M, allowing the goroutine to be
rescheduled during software floating point simulation
would mean some other goroutine could overwrite the registers
or perhaps the goroutine would continue running on a different
M entirely.

Disallow preemption during the software floating point
routines to make sure that a function full of floating point
instructions has the same floating point registers throughout
its execution.

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/12298043