remove stuff that doesn't belong in powerpc 32-bit sigcontext

these fields were wrongly copied from the kernel's ppc64 struct def
fix up leftover, incorrect NSIG definitions in arch-specific signal.h
add missing startfiles for powerpc
fix powerpc setjmp/longjmp to save/restore float regs; enlarge/align jmp_buf
fix powerpc sigsetjmp asm to match the new jmp_buf size/offsets
sigcontext/mcontext cleanup for arch-specific bits

with these changes, the members/types of mcontext_t and related stuff
should closely match the glibc definitions. unlike glibc, however, the
definitions here avoid using typedefs as much as possible and work
directly with the underlying types, to minimize namespace pollution
from signal.h in the default (_BSD_SOURCE) profile.

this is a first step in improving compatibility with applications
which poke at context/register information -- mainly debuggers, trace
utilities, etc. additional definitions in ucontext.h and other headers
may be needed later.

if feature test macros are used to request a conforming namespace,
mcontext_t is replaced with an opaque structure of the equivalent size
and alignment; conforming programs cannot examine its contents anyway.
fix termios constants on mips
fix powerpc ucontext, again..

it should now really match the kernel. some of the removed padding
corresponded to the difference between user and kernel sigset_t. the
space at the end was redundant with the uc_mcontext member and seems
to have been added as a result of misunderstanding glibc's definition
versus the kernel's.
begin sys/user.h and sys/reg.h fixes for ports

aside from microblaze, these should be roughly correct for all archs
now. some misc junk macros and typedefs are missing, which should
probably be added for max compatibility with trace/debug tools.
eliminate gdb complaints about missing linux-gate.so.1

actually, the hard-coded name should be eliminated too, and replaced
by a search for the soname in the headers, but that can be done
separately later.
remove microblaze user.h incorrectly copied from arm
make sys/procfs.h mostly work on most archs

these structures are purely for use by trace/debug tools and tools
working with core files. the definition of fpregset_t, which was
previously here, has been removed because it was wrong; fpregset_t
should be the type used in mcontext_t, not the type used in
ptrace/core stuff.
fixup mcontext stuff to expost gregset_t/fpregset_t as appropriate
fix missing limits when only _BSD_SOURCE is defined

the missing check did not affect the default profile, since it has
both _XOPEN_SOURCE and _BSD_SOURCE defined, but it did break programs
which explicitly define _BSD_SOURCE, causing it to be the only feature
test macro present.
fix eventfd and inotify nonblock/cloexec flags to match arch values
remove stat member aliases from ppc-specific bits/stat.h

if these are to be supported, they belong in the main stat.h, not
repeated for each arch.
update readme and release notes for 0.9.8
fix some restrict-qualifier mismatches in newly added interfaces

these should have little/no practical impact but they're needed for
strict conformance.
fix ordering of shared library ctors with respect to libc init

previously, shared library constructors were being called before
important internal things like the environment (extern char **environ)
and hwcap flags (needed for sjlj to work right with float on arm) were
initialized in __libc_start_main. rather than trying to have to
dynamic linker make sure this stuff all gets initialized right, I've
opted to just defer calling shared library constructors until after
the main program's entry point is reached. this also fixes the order
of ctors to be the exact reverse of dtors, which is a desirable
property and possibly even mandated by some languages.

the main practical effect of this change is that shared libraries
calling getenv from ctors will no longer fail.
provide NSIG under _BSD_SOURCE (default) as well as _GNU_SOURCE

this fixes a regression related to the changes made to bits/signal.h
between 0.9.7 and 0.9.8 that broke some (non-portable) software.