linux: Fix declaration of rt_sigreturn() on arm64

On Linux, this syscall doesn't take any arguments; instead
it assumes the context was put on the stack.

Reviewed By: dchagin
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D31251

(cherry picked from commit 2561da0636c1b8504d05246258a5333b621a2433)