Add gc_debug build

[yuyichao]

I use the nightly_llvmsvn build as a template to create this one. Right now this only enable the WITH_GC_DEBUG_ENV option in order to test if anything breaks the compilation of the gc debugging code and to check if anything obvious is broken in the test.

Ideally, there should be another bot that use the same build option (either building it or take it from this bot) and run the test suite with more aggressive GC collection.

[staticfloat]

Do you need to build a custom LLVM with this build? If not, you can remove the lines cleaning out llvm.

[yuyichao]

No I don't.

And done.

[staticfloat]

Cool; so this will run twice a day; once at midnight, once at noon. Is that what you want?

[yuyichao]

This is the one to make sure no one breaks the debug option so it should be relatively fast. I'm fine all the way from twice a day to every two days so I just left the llvm-nightly settings. I can certainly change it if you prefer lower frequency (since it's not like we'll have too many commit to go through in a day or two if someone actually breaks it.).

[staticfloat]

Great. Thanks! Let's see how this goes. I might rotate the hours so that all our nightly stuff isn't happening at the same time, but I can do that after-the-fact.

[staticfloat]

It works! And it's not even that much slower than a regular build on the same machine. :)