mk: clean-llvm for cross-compile
auto merge of #7492 : yichoi/rust/fix_cleanllvm, r=luqmana

fix clean-llvm in mk/clean.mk for cross-compile

after #7442 landed, below error produced while android cross-compile
```
make[1]: *** No rule to make target `clean-llvmarm-linux-androideabi', needed by `clean-llvm'.  Stop.
make[1]: Leaving directory `/home/yichoi/rust_latest/build'
make: *** [rustllvm/llvm-auto-clean-stamp] Error 2
```