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 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.

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

Approved by: re (implicit)

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)