Reactor: Expose Capabilities.

Subzero is now lagging behind LLVM in terms of functionality.
Stub a new Capabilities, pre-populated with the Call() feature (implemented in a follow-up change).

With this we can skip tests that are known to not pass.

Bug: b/130746922
Change-Id: I033b63034a6dccd31418b1d1898114adbd718025
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/29409
Presubmit-Ready: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>

src/Reactor/Reactor.hpp

diff --git a/src/Reactor/Reactor.hpp b/src/Reactor/Reactor.hpp
index 7846711..e6f3ff2 100644
--- a/src/Reactor/Reactor.hpp
+++ b/src/Reactor/Reactor.hpp
@@ -34,6 +34,12 @@
 
 namespace rr
 {
+	struct Capabilities
+	{
+		bool CallSupported; // Support for rr::Call()
+	};
+	extern const Capabilities Caps;
+
 	class Bool;
 	class Byte;
 	class SByte;