The V8 GYP file now uses its own target architecture. It default to the standard target_arch, but can be set to a separate value. E.g. using

  export GYP_DEFINES="target_arch=ia32 v8_target_arch=arm"

makes it possible to have the V8 ARM simulator be used in a IA32 build.

Added some checking of supported host/target architecture combinations.
Review URL: http://codereview.chromium.org/1790001

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4490 ce2b1a6d-e550-0410-aec6-3dcde31c8c00