Through using ndk-build, -Wno-unused-command-line-argument is passed in
somewhere in the pipeline and hides this warning.
The warning can be turned on with:
APP_CPPFLAGS := -Wunused-command-line-argument ...
and then when building, you can see:
C/C++: clang++: warning: argument unused during compilation: '-mfloat-abi=softfp' [-Wunused-command-line-argument]
C/C++: clang++: warning: argument unused during compilation: '-mfpu=neon' [-Wunused-command-line-argument]
C/C++: clang++: warning: argument unused during compilation: '-maes' [-Wunused-command-line-argument]
These are unused because both floating-point and NEON are required in
all standard ARMv8 implementations. [1] [2]
[1] https://developer.arm.com/documentation/den0024/a/AArch64-Floating-point-and-NEON
[2] https://stackoverflow.com/a/29891469
Since NDKr15 (released 2017), unified headers are used by default [1]
Remove -isystem option that was passing bad values to command-line.
The actual value being passed to command-line was:
```
-isystem DK/sysroot/usr/include/RIPLE
```
because of using $NDK and $TRIPLE instead of $(NDK) and $(TRIPLE)
But regardless, $NDK and $TRIPLE were never actually defined values and were just
place-holders mentioned in [1]
[1] https://android.googlesource.com/platform/ndk/+/ndk-release-r16/docs/UnifiedHeaders.md
