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

riscv64-linux-gnu- matches the triple used by Debian's package.

Change-Id: Ia05652a44fdca4ed26ce42521e047d5b57ebbcdf
diff --git a/build/cmake/toolchains/riscv-linux-gcc.cmake b/build/cmake/toolchains/riscv-linux-gcc.cmake
index 65b7c59..5cdc9e4 100644
--- a/build/cmake/toolchains/riscv-linux-gcc.cmake
+++ b/build/cmake/toolchains/riscv-linux-gcc.cmake
@@ -18,7 +18,7 @@
 if("${CROSS}" STREQUAL "")
 
   # Default the cross compiler prefix to something known to work.
-  set(CROSS riscv64-unknown-linux-gnu-)
+  set(CROSS riscv64-linux-gnu-)
 endif()
 
 if(NOT CMAKE_C_COMPILER)