compiletest: Fix a couple of test re-run issues
compiletest: Fix a couple of test re-run issues

* Re-run rustdoc tests if rustdoc or htmldocck.py was updated.
* Put stamp files in the correct subdirectories to avoid clashes when
the file names match but the subdirectory doesn't.

GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust