Clean up some A64 specific code in common code that was introduced by A64 merge

 - Remove arch specific check macros
 - Remove duplicate code in code-stubs.h
 - Remove flag check in bootstrap.cc which was introduced for A64 bringup
 - Remove A64 specific test message expectations

R=rodolph.perfetta@arm.com, ulan@chromium.org

Review URL: https://codereview.chromium.org/134333011

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19325 ce2b1a6d-e550-0410-aec6-3dcde31c8c00