When kernel CONFIG_KASAN is enabled, the function memset becomes
a global function __memset. The bcc application with probe
rewrite will incur an error like:

  error: <unknown>:0:0: in function tracepoint__task__task_newtask i32
         (%struct.tracepoint__task__task_newtask*): A call to global function
         '__memset' is not supported. Only calls to predefined BPF helpers
         are allowed.

Replace memset function with clang __builtin_memset so that
the compiler is able to eventually optimize it away.
Signed-off-by: Yonghong Song <yhs@localhost.localdomain>