As it is currently written, CallLambdaTrampoline does not return a
value. However, some of the functions that are being wrapped may
return a value that the JIT is expected to understand. A compiler
*cough cough clang* may opt to alter %rax after the wrapped lambda
returns, e.g. popping a previous value, which can clobber the
return value. If we actually have a return value, then the compiler
must not clobber it.

Requires a minor change to in the JIT to make sure everything still works.

Previously on FPR pushing and popping we would do a single STR/LDR per quad FPR we wanted to push/pop.
In most of our cases when we are pushing and popping VFP registers they will be consecutive registers that will save more efficiently using the NEON
loadstores that can do up to four quad registers.
So this can potentially cutting instructions down to ~1/4th the amount of instructions if the registers are all consecutive.

On the Cortex-A57 this is basically just an icache improvement, but on the Nvidia Denver this may be optimized to be more efficient. Either way it's a
win.