ppc-linux-gcc.cmake: update default CROSS value

powerpc64le-linux-gnu- matches the triple used by Debian's package.

Change-Id: I6c24440d12365b9c848c10d4b478bb7182eebe6a
diff --git a/build/cmake/toolchains/ppc-linux-gcc.cmake b/build/cmake/toolchains/ppc-linux-gcc.cmake
index aeb2420..5c5dfe5 100644
--- a/build/cmake/toolchains/ppc-linux-gcc.cmake
+++ b/build/cmake/toolchains/ppc-linux-gcc.cmake
@@ -18,7 +18,7 @@
 if("${CROSS}" STREQUAL "")
 
   # Default the cross compiler prefix to something known to work.
-  set(CROSS powerpc64le-unknown-linux-gnu-)
+  set(CROSS powerpc64le-linux-gnu-)
 endif()
 
 if(NOT CMAKE_C_COMPILER)