Previously I've been using the Linaro toolchains and just building
static binaries. However, the Linaro toolchains have a broken
pthread_rwlock_wrlock—it does nothing and then unlocking corrupts the
lock.

Building with the Android NDK avoids this.

These build instructions depend on
https://github.com/taka-no-me/android-cmake which people will need to
clone into util/ if they want to use the Android NDK.

Change-Id: Ic64919f9399af2a57e8df4fb4b3400865ddb2427
Reviewed-on: https://boringssl-review.googlesource.com/4600
Reviewed-by: Adam Langley <agl@google.com>