Revert r276117 "[XRay] Basic initialization and flag definition for XRay runtime"

and also the follow-up "[xray] Only build xray on Linux for now"

Two build errors were reported on the llvm-commits list:

	[ 88%] Building CXX object lib/xray/CMakeFiles/clang_rt.xray-x86_64.dir/xray_flags.cc.o
	/mnt/b/sanitizer-buildbot1/sanitizer-x86_64-linux/build/llvm/projects/compiler-rt/lib/xray/xray_init.cc:23:10: fatal error: 'llvm/Support/ELF.h' file not found
  #include "llvm/Support/ELF.h"
					 ^

and

	In file included from /w/src/llvm.org/projects/compiler-rt/lib/xray/xray_interface.cc:16:
	/w/src/llvm.org/projects/compiler-rt/lib/xray/xray_interface_internal.h:36:8: error:
				no type named 'size_t' in namespace 'std'
		std::size_t Entries;
		~~~~~^

llvm-svn: 276186