Use ARM64 WARP on ARM64 machines (microsoft#7862)

It turns out that our build agents report their processor as AMD64,
which meant that the previous logic in DownloadWarp.cmake would use the
AMD64 version of WARP. This change fixes that.

Author: damyanp

tools/clang/test/taef_exec/DownloadWarp.cmake

@@ -76,7 +76,7 @@ endif()
 
 set(WARP_DIR ${CMAKE_BINARY_DIR}/nuget/${WARP_PACKAGE})
 
-if(${CMAKE_SYSTEM_PROCESSOR} STREQUAL "ARM64")
+if((${CMAKE_SYSTEM_PROCESSOR} STREQUAL "ARM64") OR ("${CMAKE_C_COMPILER_ARCHITECTURE_ID}" STREQUAL "ARM64"))
     set(WARP_ARCH "arm64")
 elseif ((CMAKE_GENERATOR_PLATFORM STREQUAL "Win32") OR ("${CMAKE_C_COMPILER_ARCHITECTURE_ID}" STREQUAL "X86"))
     set(WARP_ARCH "win32")