Dispatcher trampoline should preserve contents of VFP registers about mambo

[lgeek]

The workaround I've been using is to build MAMBO without NEON/VFP support. It should also also be linked against a NEON/VFP-free stdlib implementation.

However, you are right that the proper solution is to preserve the value of the VFP registers (note to self, also the value of FPSCR). I'll look into how to support VFP versions with either 16 or 32 registers, but I'm not too fussed about supporting processors with no VFP because as far as I know it's implemented in all ARMv7-A cores. By the way, d8-d15 are callee-saved, so you shouldn't have to save & restore them.

[lgeek]

This was fully fixed by the patch from April 2017, however that assumes an implementation with 32 double precision FP registers. At the time I thought it might be useful to maintain compatibility with 16 FP register implementations by adding some sort of runtime detection. However, the NEON extension requires 32 FP registers and we're already building MAMBO assuming NEON is supported. Because the only ARMv7A processor without NEON support is Tegra2, we're not going to bother supporting implementations without NEON support or with only 16 FP registers.