Enable ignoring tests based on architecture.

Now, tests can only be ignored on OS basis. However, some tests - e.g., compile-fail/asm-misplaced-option.rs - should be skipped because of the target architecture. 

(Note: in the case of compile-fail/asm-misplaced-option.rs, #[cfg(any(target_arch = "x86", target_arch = "x86_64"))] pub fn main() prevents the Intel assembly to be processed on another architecture but does not prevent compiletest to find and expect the warnings within main.)

So, this PR proposes to enhance compiletest to recognize // ignore-ARCH lines as well.
Because `x86_64` contains `x86`, doesn't this mean that `// ignore-x86` will ignore 32 and 64 bit platforms?