NDK r21e released

[Dan Albert]

NDK r21e is now available in the stable channel of the SDK manager (use ndkVersion "21.4.7075529" in your build.gradle) and for download from our website.

Includes fixes for the following issues:

• Issue 1248: Fixed LLVM codegen segfault that occurred for some Neon intrinsics.
• Issue 1267: Fixed `APP_BUILD_SCRIPT` not working when the file was named anything other than `Android.mk`.
• Issue 1281: Fixed missing `<stdbool.h>` includes in some headers that caused issues for inclusion from C sources.
• Issue 1307: Protect against incompatibilities between NDK Binder API and AIDL. Note that in the future the AIDL support headers will be moved from the NDK to build-tools to remove the potential for incompatibility.
• Issue 1370: Fixed missing datetime import when building the shaderc library.
• Issue 1388: Fixed LLVM's incorrect conversion to list files for long command lines on Windows.

Note that this is likely our final release of the r21 LTS release, as the next major release of the NDK (r23) will be a new LTS release. The release schedule for r23 is not yet certain so an r21f remains possible, but once r23 is released to stable the support window for r21 will end.