Subzero: Add a separate check-xtest target to Makefile.standlone.
Subzero: Add a separate check-xtest target to Makefile.standlone.

This makes it more convenient to directly test the crosstests without having to go through the lit tests first.

BUG= none
R=kschimpf@google.com

Review URL: https://codereview.chromium.org/1005163002

bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader