Pass flags and target ISAs to filetests.

Add a `needs_isa()` method to the SubTest trait, and pass a TargetIsa
trait object to those sub-tests that request it.

When multiple sub-tests and ISAs are specified, test the cross product.

If a sub-test requires an ISA, but none are specified, fail the test. In
the future, it may be a good idea to generate a default set of ISAs and
test against those.

src/libreader/lib.rs

@@ -9,6 +9,7 @@ pub use error::{Location, Result, Error};
 pub use parser::{parse_functions, parse_test};
 pub use testcommand::{TestCommand, TestOption};
 pub use testfile::{TestFile, Details};
+pub use isaspec::IsaSpec;
 pub use sourcemap::SourceMap;
 
 mod error;