Support Android NDK 18+ (#1659)

[BugsBeGone]

Backwards compatible changes to native makefile, such that ancient GCC-based toolchains can still be used, but also allow newer LLVM-based tools from NDK 18 onwards.

When building with anything available from the Android SDK manager currently, all that should be necessary is to set $NDK to the top-level path, and $NDK_PLATFORM to the target SDK level. If no Pkg.Revision can be found, you will also need to set $NDK_VER to the major version.

Checks for armv5 & MIPS target architectures are also performed, which are not supported from NDK 17,

[BugsBeGone]

Forgot to mention, I have no opinion whatsoever about whether the shipped binaries should actually be upgraded or not, I understand that testing anything like this is extremely tedious at the best of times. This is purely intended to help those wanting to build jnidispatch.so from source, who may at some point be unable to use or even obtain ancient NDK toolchains.