Tidy up how __sigaction14_sigtramp.c is added to SRCS.

Decorate the x86_64 signal trampoline with CFI attributes easing unwinding

Combine the approach provided by Nikhil Benesch and Andrew Cagney.

Now, the unwinders (in gccgo, backtrace(3), etc) can unwind properly
the stack from a signal handler.

Fixes lib/55719 by Nikhil Benesch

Don't include sigtramp or foocontext routines in rumprun mode. They won't
work (since a rump kernel does not provide NetBSD _lwp syscalls nor
signals), but they cause unwanted symbols to be pulled in when the
resulting libc is linked.

problem reported by Robert Gifford on rumpkernel-users

G/c KMSRCS and KMINCLUDES now that we have common/lib/libc

kill sigcontext passing to signal handlers, we don't have to worry
about compatibility

Userland amd64 siginfo support.

Enable the libc-provided signal trampoline. The old __sigaction14
entry point has been replaced by the new __sigaction14() which is a
wrapper around the __sigaction_sigtramp system call (which is itself
not directly exposed outside of libc).

No shared library version change is necessary; no new symbols
are exposed outside of libc.

Initial commit of x86-64 specific parts of libc.