Teach Makefile to build LayoutTestRelay when building for iOS Simulator

Teach Makefile to build LayoutTestRelay when building for iOS Simulator
https://bugs.webkit.org/show_bug.cgi?id=150849

Reviewed by Alexey Proskuryakov.

.:

Add support for overriding the user-provided arguments SDKROOT and ARCHS
on a per Makefile basis.

* Makefile.shared:

Tools:

Override the user-specified arguments SDKROOT and ARCHS to use the default SDK
and default ARCHS since LayoutTestRelay is a Mac command line tool.

* LayoutTestRelay/Makefile: Define OVERWRITE_SDKROOT and OVERWRITE_ARCHS.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@192106 268f45cc-cd09-0410-ab3c-d52691b4dbfc