portability: platform compiler flags

-mcx16 and -march=native are platform specific.  Leave them out if we
aren't targeting chips that need it.

CMakeLists.txt

@@ -72,7 +72,12 @@ if(NOT MSVC)
   if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
     target_link_libraries(snmalloc_lib INTERFACE atomic)
   endif()
-  target_compile_options(snmalloc_lib INTERFACE -mcx16)
+  if(${CMAKE_SYSTEM_PROCESSOR} STREQUAL "x86_64")
+   target_compile_options(snmalloc_lib INTERFACE -mcx16)
+  elseif(${CMAKE_SYSTEM_PROCESSOR} STREQUAL "x86")
+   target_compile_options(snmalloc_lib INTERFACE -mcx16)
+  # XXX elseif ARM?
+  endif()
 else()
   set(WIN8COMPAT FALSE CACHE BOOL "Avoid Windows 10 APIs")
   if (WIN8COMPAT)
@@ -115,7 +120,13 @@ if(NOT DEFINED SNMALLOC_ONLY_HEADER_LIBRARY)
     set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} /Zi")
     set(CMAKE_EXE_LINKER_FLAGS_RELEASE "${CMAKE_EXE_LINKER_FLAGS_RELEASE} /DEBUG")
   else()
-    add_compile_options(-march=native -fno-exceptions -fno-rtti -g -ftls-model=initial-exec -fomit-frame-pointer)
+    add_compile_options(-fno-exceptions -fno-rtti -g -ftls-model=initial-exec -fomit-frame-pointer)
+    if(${CMAKE_SYSTEM_PROCESSOR} STREQUAL "x86_64")
+     add_compile_options(-march=native)
+    elseif(${CMAKE_SYSTEM_PROCESSOR} STREQUAL "x86")
+     add_compile_options(-march=native)
+    # XXX elseif ARM?
+    endif()
   endif()
 
   macro(subdirlist result curdir)