Fix g++ compile option error on ppc64le and riscv64.

ppc64le: `unrecognized command-line option '-march=native'`
riscv64: `'-march=native': ISA string must begin with rv32 or rv64`

--- a/cmake/DrawpileCompilerOptions.cmake
+++ b/cmake/DrawpileCompilerOptions.cmake
@@ -83,8 +83,14 @@
 	endif()
 
 	if(ENABLE_ARCH_NATIVE)
-		add_compile_options(-march=native)
-	endif()
+    if (CMAKE_SYSTEM_PROCESSOR MATCHES "ppc64le")
+      add_compile_options(-mcpu=powerpc64le)
+    elseif(CMAKE_SYSTEM_PROCESSOR MATCHES "riscv64")
+      add_compile_options(-march=rv64id)
+    else()
+      add_compile_options(-march=native)
+    endif()
+  endif()
 
 	# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105725
 	if(NOT CMAKE_CXX_COMPILER_ID STREQUAL "GNU" OR CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 12.2)