remap_neon: use register r12 instead of r7

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[1] this fix, all I had to do was to
test that it works. The code builds now, and cpu-remap-test also
succeeds.

[1] https://lists.openembedded.org/g/openembedded-core/message/136786