Update to NDK version 26

[mhsmith]

As mentioned in PEP 738, we should use the current long-term support version of the NDK. I quickly tried updating it yesterday and hit a couple of issues, both of which should be easy to fix:

• The assembler path has changed because they've switched from binutils to LLVM.

• libffi fails to build due to a stricter compiler (fixed in a newer version).