Only generate offline asm for the ARCHS (xcodebuild) or the current system (CMake)

Only generate offline asm for the ARCHS (xcodebuild) or the current system (CMake)
https://bugs.webkit.org/show_bug.cgi?id=175690

Reviewed by Michael Saboff.

This should reduce some of the time we spend building offline asm
in our builds (except for linux since they already did this).

* CMakeLists.txt:
* JavaScriptCore.xcodeproj/project.pbxproj:
* offlineasm/backends.rb:
* offlineasm/generate_offset_extractor.rb:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@220994 268f45cc-cd09-0410-ab3c-d52691b4dbfc