rustbuild: make a config.toml entry for codegen tests

[durka]

If you configure for system LLVM and run bootstrap.py without first running ./configure --disable-codegen-tests, the sanity check panics:

thread 'main' panicked at 'filecheck executable "/root/rust/FileCheck" does not exist', src/bootstrap/sanity.rs:93

(where /root/rust is the directory part of the path to llvm-config given in config.toml)

Per @alexcrichton there should be an option in config.toml to disable the codegen tests.

[alexcrichton]

Should be a pretty easy bug to tackle! What needs to happen here is:

• An option needs to be added to this struct
• That option then needs to configure the global option
• That configuration would probably happen in this block.

[durka]

I'll take it!