Ensure the system arch is lowercase.

On Windows, `CMAKE_SYSTEM_PROCESSOR` can return AMD64, which fails the
current check.

Author: FtZPetruska

cmake/GetTriplet.cmake

@@ -1,5 +1,5 @@
 function(get_host_triplet triplet)
-    set(host_arch ${CMAKE_SYSTEM_PROCESSOR})
+    string(TOLOWER "${CMAKE_SYSTEM_PROCESSOR}" host_arch)
     if(host_arch STREQUAL "x86")
         set(host_arch "i686")
     elseif(host_arch STREQUAL "amd64")
@@ -29,7 +29,7 @@ function(get_host_triplet triplet)
 endfunction()
 
 function(get_build_triplet triplet)
-    set(host_arch ${CMAKE_HOST_SYSTEM_PROCESSOR})
+    string(TOLOWER "${CMAKE_HOST_SYSTEM_PROCESSOR}" host_arch)
     if(host_arch STREQUAL "x86")
         set(host_arch "i686")
     elseif(host_arch STREQUAL "amd64")