<sys/signal.h>: Adjust the type of stack_t's ss_sp from char * to void *.

libc: Add prototypes for various functions we use in __weak_reference().

<sys/cdefs.h>: Rework __weak_reference() macro.

My LTO build blew away weak symbols from slim LTO objects.
Use __strong_reference() + weak attribute to allow the compiler to catch
extern declaration

<sys/cdefs.h>: Rework __weak_reference() macro.

My LTO build blew away weak symbols from slim LTO objects.
Use __strong_reference() + weak attribute to allow the compiler to catch
extern declarations and not to fold weak symbols as local ones.

Keep previous version as __weak_reference_asm() in _pthread_stubs.c for
now due to several issues (there is a need to do it in a cleaner way).

lib/libc/gen/ucontext.c:
add missing __DECONST for ucp, shouldn't sigreturn take const ucontext_t?

lib/libc/inet/inet_ntoa.c:
add missing #undef inet_ntoa_r

No symbol changes in libc and librt on normal compilation.

While there, add __weak_symbol attribute for future additions.

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libc - Fix bugs in getcontext(), setcontext(), and swapcontext()

* Fix multiple bugs revealed by qemu's use of these functions. Most of
these fixes are accomplished by calling sigreturn(uctx) to

libc - Fix bugs in getcontext(), setcontext(), and swapcontext()

* Fix multiple bugs revealed by qemu's use of these functions. Most of
these fixes are accomplished by calling sigreturn(uctx) to restore the
state instead of trying to roll our own in userland. This won't be much
slower (if at all) because we had to save and restore the signal state
in the userland code anyway, so we could not avoid making at least one
system call.

Using sigreturn() handles the signal mask atomicy for us so we don't have
to deal with it and fixes numerous other issues. Along with this change,
adjust getcontext() and makecontext() to fill out additional important
fields in the ucontext that sigreturn() inspects.

* Fixes two stack corruption bugs. First, getcontext() was calling
get_mcontext() and get_mcontext() was setting up the setcontext return
state to return 1 ... from get_mcontext(), NOT from getcontext(). If
normal operations or signals mess with the stack, the double return
will not work. Oops.

Secondly, getcontext scribbled over the red-zone in a way that is not
permitted.

* setcontext() was restoring the context as saved by makecontext() or
getcontext(), but setcontext() can also be called with the ucontext
from the signal handler which requires a full restore. setcontext()
was not restoring FPU or scratch registers or rflags.

* Fixes signal restoration bug and corruption that can mess up emulation
in qemu.

* Fixes issues with qemu related to SMP startup and lack of preemption.

Reported-by: ivadasz

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libc: correct copyright notice in makecontext.c

libc: add getcontext, setcontext, makecontext and swapcontext on x86_64

The *context functions are not part of the POSIX standard anymore, but
are still used by quite some packages.