Use sa_sigaction signal handlers on Linux. The glibc definition of

sigset_t is different from the one used in the Linux kernel.  This means
that we cannot make assignments of sigset_t objects without corrupting
memory.  To work around this problem we update uc_sigmask differently
on Linux than on other systems.  Also, remove x87 control word updates
since the x87 is setup correctly inside call_into_lisp.