AP_Scripting: stash and restore FPU register context in LUAI_TRY

[tridge]

this fixes an issue where a lua library function triggers an exception after it does a math operation which changes the floating point registers on M7 MCUs (such as STM32H7). An example is math.random() which calls math_random(), which pre-calculates a double value before checking if the arguments to the call are valid. When it then checks and finds invalid values the exception longjmp does not restore the floating point registers.

An example script which reproduces this error is:
math.random(1,0)

if you run that and then do a "scripting restart" scripting dies

This PR takes a belt and braces approach to fixing the problem:

• it adds the FPU save/restore in the LUAI_TRY() code
• it avoids the use of FPU registers in the thread loop by changing to O0 optimisation

either change is sufficient to fix the issue

many thanks to @bugobliterator for tracking this down and @IamPete1 for finding the issue originally

[IamPete1]

Just retested on CubeOrangePlus. Still works.

I wonder if we should fix the random method as well?