…roideabi` rlib

I ran `./x.py dist --host= --target=armv7-linux-androideabi` before this diff:
```
$ nm build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/armv7-linux-androideabi/lib/libcompiler_builtins-3d9661a82c59c66a.rlib 2> /dev/null | grep __sync_fetch_and_add_4 | wc -l
2
```
And after:
```
$ nm build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/armv7-linux-androideabi/lib/libcompiler_builtins-ffd2745070943321.rlib 2> /dev/null | grep __sync_fetch_and_add_4 | wc -l
1
```
Fixes rust-lang#93310

This disables incremental compilation by default and adds a snippet to the
compiler release notes explaining the rationale and encouraging testing.