Run tests across many (configurable) compilers

[stephenrkell]

Currently we use as our C compiler ${CC:-cc}, in effect. However, some test cases are special-cased to use clang. Let's instead make the test cases configurable and at least look for gcc and clang by default.

I don't want to do this until we've sped up the test cases, however. That means:

• lower compiler wrapper overhead (I'm working on this, in toolsub)
• lower metadata build times (@mbyzhang has some work pending on this, and the fabled libdw switchover should have additional impact when it happens)
• lower startup overhead (Reduce startup delays caused by libsystrap #123)

In short, I think we will have enough progress on all of these soon-ish that doing a multi-compiler run of the tests will not be painfully slow. Proof will be in the pudding, however....