Fix call to rt_sigaction

Fix call to rt_sigaction

Despite the fact that many places imply that sigaction and rt_sigaction
are essentially the same, rt_sigaction's signature is actually
different-- it takes the size of the kernel's sigset_t as an extra argument.

This change uses the kernel_sigaction, kernel_sigset_t and sys_rt_sigaction
provided by linux_syscall_support.h instead of rolling our own.

BUG=473973

[cjhopman]

mark: *

One more for ya.

[Mark Mentovai]

LGTM. Good luck.