Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r251047:
Avoid a nested frame for getcontext() call in check_deferred_signal().

MFC r234115:
Do not restore the register holding the TLS pointer when doing various
usermode context switches (long jumps and ucontext operations). If these
are used across threads, multiple threads can end up with the same TLS base.
Madness will then result.

This makes behavior on PPC match that on x86 systems and on Linux.

MFC r234003:

Properly resolve the _ctx_start function descriptor (the symbol _ctx_start
is a descriptor, not a code address), which prevents crashes when starting
a context. This fixes QEMU on powerpc64.

MFC r230429:
Add API for obtaining extended machine context states that cannot be
fit into existing mcontext_t.

On i386 and amd64 do return the extended FPU states using
getcontextx(3). For other architectures, getcontextx(3) returns the
same information as getcontext(2).

MFC r230864:
Make the sys/ucontext.h self-contained by changing the return type
of __getcontextx_size(3) from size_t to int.

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Fix typos - remove duplicate "the".

PR: bin/154928
Submitted by: Eitan Adler <lists at eitanadler.com>
MFC after: 3 days

Turn off default generation of userland dot symbols on powerpc64 now that
we have a binutils that supports it. Kernel dot symbols remain on to assist
DDB.

Mark assembler sources from libc as working with non-executable stack.

Reviewed and tested by: nwhitehorn

Provide 64-bit PowerPC support in libc.

Obtained from: projects/ppc64