Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

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

Fix powerpc setjmp FPR saving/restoring.

X-MFC-With: r279784

Save and restore non-volatile floating-point registers during longjmp().
This should also save and restore non-volatile Altivec registers, but that
needs to wait on solving two problems:
1. Adding the nonvolatile vector registers means we need 5 more than _JBLEN
entries in jmp_buf on 32-bit targets (64-bit is OK).
2. Need to figure out how to determine if saving/restoring vector regs
is supported on the current CPU from userland.

MFC after: 1 month

Ensure that every ENTRY(foo) has a matching END(foo).
It allows to build with newer binutils

Differential Revision: https://reviews.freebsd.org/D877
Reviewed by: jhibbits

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 after: 10 days

Replace the assembler macro WEAK_ALIAS with a new macro WEAK_REFERENCE which
has the same API as __weak_reference(). Give 'x' in SYS.h a more meaningful
name.

Tested on 32- and 64-bit PowerMac.

Reviewed by: bde

Two other places where we can use WEAK_ALIAS.

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

Reviewed and tested by: nwhitehorn

Minimal libc for PowerPC.

Reviewed by: benno

Approved by: re (blanket)