If we are using vfork, and if something (such as TSAN) is intercepting
the sigaction function, then we must call the system call, not the
libc function. Otherwise the intercepted sigaction call in the child
may trash the data structures in the parent.

Change-Id: Id9588bfeaa934f32c920bf829c5839be5cacf243
Reviewed-on: https://go-review.googlesource.com/50251Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Austin Clements <austin@google.com>