SERVER-4683 Only use embedded tcmalloc on x86_64 linux by default.

SConstruct

@@ -203,7 +203,8 @@ add_option( "clang" , "use clang++ rather than g++ (experimental)" , 0 , True )
 # debugging/profiling help
 
 add_option( "allocator" , "allocator to use (tcmalloc or system)" , 1 , True,
-            default=(sys.platform.startswith('linux') and 'tcmalloc' or 'system') )
+            default=((sys.platform.startswith('linux') and (os.uname()[-1] == 'x86_64')) and
+                     'tcmalloc' or 'system') )
 add_option( "gdbserver" , "build in gdb server support" , 0 , True )
 add_option( "heapcheck", "link to heap-checking malloc-lib and look for memory leaks during tests" , 0 , False )
 add_option( "gcov" , "compile with flags for gcov" , 0 , True )