Make rust core use NDK >= r23

[GhenadiePusca]

So this is workaround to allow building core with any NDK version >= r23. This is a little bit of magic I didn't want to do initially, but it seems that is quite safe(many projects also adopted this kind of solution).

The root cause of the issue is that newer NDK versions use -lunwind instead of -libgcc, while rust building system seems to be using still the version with -libgcc

What changed:

• core Make will no more download the NDK, instead it will use the system one.
• Trick rust into using -lunwind as -libgcc

[GhenadiePusca]

Needs more work, the proposed workaround works only with r23 and r24, but not with r25... In the end it is not about NDK version, I did had a faulty test.

[JJ]

Looks fine, just the usual questions to understand it a bit better.

[GhenadiePusca]

GH action fails mostly due to GH env still using obsolete ndk-bundle. However GH is about to remove it, probably in favour of normal android NDK -> actions/runner-images#5879.

Will postpone this PR until GH makes all the changes on the env, which will happen on Monday 24th.

[JJ]

Looks great. The .fbs file was simply left over, right?

[GhenadiePusca]

Looks great. The .fbs file was simply left over, right?

Right.
What I have to do to complete this, is to check if NDK r25 can be used. I know that GH env did add r25, and it should be the default, but it is not for some reason.