remap_neon: use register r12 instead of r7
When the Thumb instructions set is used and frame pointers are enabled (-fno-omit-frame-pointer), r7 can't be used, because it's used for the frame pointer. Trying to use r7 caused the compilation to fail.
Thanks to Andre McCurdy for suggesting this fix, all I had to do was to test that it works. The code builds now, and cpu-remap-test also succeeds.