aarch64: Re-implement setcontext without rt_sigreturn syscall

The current implementation of setcontext uses rt_sigreturn to restore
the contents of registers. This contrasts with the way most other
architectures implement setcontext:

  powerpc64, mips, tile:

  Call rt_sigreturn if context was created by a call to a signal handler,
  otherwise restore in user code.

  powerpc32:

  Call swapcontext system call and don't call sigreturn or rt_sigreturn.

  x86_64, sparc, hppa, sh, ia64, m68k, s390, arm:

  Only support restoring "synchronous" contexts, that is contexts
  created by getcontext, and restoring in user code and don't call
  sigreturn or rt_sigreturn.

  alpha:

  Call sigreturn (but not rt_sigreturn) in all cases to do the restore.

The text of the setcontext manpage suggests that the requirement to be
able to restore a signal handler created context has been dropped from
SUSv2:

  If  the context was obtained by a call to a signal handler, then old
  standard text says that "program execution continues with the program
  instruction following the instruction interrupted by the signal".
  However, this sentence was removed in SUSv2, and the present verdict
  is "the result is unspecified".

Implementing setcontext by calling rt_sigreturn unconditionally causes
problems when used with sigaltstack as in BZ #16629. On this basis it
seems that aarch64 is broken and that new ports should only support
restoring contexts created with getcontext and do not need to call
rt_sigreturn at all.

This patch re-implements the aarch64 setcontext function to restore
the context in user code in a similar manner to x86_64 and other ports.

ChangeLog:

2014-04-17  Will Newton  <will.newton@linaro.org>

	[BZ #16629]
	* sysdeps/unix/sysv/linux/aarch64/setcontext.S (__setcontext):
	Re-implement to restore registers in user code and avoid
	rt_sigreturn system call.
stdlib/tst-setcontext.c: Check for clobbering of signal stack

On aarch64 calling swapcontext clobbers the state of the signal
stack (BZ #16629). Check that the address and size of the signal
stack before and after the call to swapcontext remains the same.

ChangeLog:

2014-04-17  Will Newton  <will.newton@linaro.org>

	[BZ #16629]
	* stdlib/tst-setcontext.c: Include signal.h.
	(main): Check that the signal stack before and
	after swapcontext is the same.
manual/setjmp.texi: Clarify setcontext and signal handlers text

Calling setcontext from a signal handler can be done safely so
it is sufficient to note that it is not recommended.

Also mention in setcontext documentation that the behaviour of
setcontext when restoring a context created by a call to a signal
handler is unspecified.

2014-04-17  Will Newton  <will.newton@linaro.org>

	* manual/setjmp.texi (System V contexts): Add note that
	calling setcontext on a context created by a call to a
	signal handler is undefined.  Update text to note that
	setcontext from a signal handler is possible but not
	recommended.
Detect if AVX2 is usable

This patch checks and sets bit_AVX2_Usable in __cpu_features.feature.

	* sysdeps/x86_64/multiarch/ifunc-defines.sym (COMMON_CPUID_INDEX_7):
	New.
	* sysdeps/x86_64/multiarch/init-arch.c (__init_cpu_features):
	Check and set bit_AVX2_Usable.
	* sysdeps/x86_64/multiarch/init-arch.h (bit_AVX2_Usable): New
	macro.
	(bit_AVX2): Likewise.
	(index_AVX2_Usable): Likewise.
	(CPUID_AVX2): Likewise.
	(HAS_AVX2): Likewise.
Add fenv test support for AArch64.
PowerPC: Add fenv macros for long double

This patch add the missing libc_<function>l_ctx macros for long
double.  Similar for float, they point to default double versions.
PowerPC: Sync pthread_once with default implementation

This patch removes the arch specific powerpc implementation and instead
uses the linux default one.  Although the current powerpc implementation
already constains the required memory barriers for correct
initialization, the default implementation shows a better performance on
newer chips.
manual: Sort overview listing by manual order.

In the glibc manual we have a "Roadmap to the manual" section at
the end of the "Introduction" chapter.

The introductory text says "Here is an overview of the contents
of the remaining chapters of this manual.", but then proceeds to
list chapters out of order and some chapter are never referenced.

This commit reorders the overview to correctly match the manual
order.

See:
https://sourceware.org/ml/libc-alpha/2014-02/msg00823.html