Refactor sanitizer build scripts.

- Move common code from buildbot_bootstrap.sh and buildbot_fast.sh to
buildbot_functions.sh.
- Do not capitalize variable names needlessly.
- Use mkdir -p where appropriate instead of "mdkir if directory does not exist".

git-svn-id: https://llvm.org/svn/llvm-project/zorg/trunk@224520 91177308-0d34-0410-b5e6-96231b3b80d8