make brace placement in public header typedef'd structs consistent

commit befa5866ee30d09c0c96e88af2eabff5911342ea performed this change
for struct definitions that did not also involve typedef, but omitted
the latter.

fix MINSIGSTKSZ values for archs with large signal contexts

the previous values (2k min and 8k default) were too small for some
archs. aarch64 reserves 4k in the signal context for future extensions
and requires about 4.5k total, and powerpc reportedly uses over 2k.
the new minimums are chosen to fit the saved context and also allow a
minimal signal handler to run.

since the default (SIGSTKSZ) has always been 6k larger than the
minimum, it is also increased to maintain the 6k usable by the signal
handler. this happens to be able to store one pathname buffer and
should be sufficient for calling any function in libc that doesn't
involve conversion between floating point and decimal representations.

x86 (both 32-bit and 64-bit variants) may also need a larger minimum
(around 2.5k) in the future to support avx-512, but the values on
these archs are left alone for now pending further analysis.

the value for PTHREAD_STACK_MIN is not increased to match MINSIGSTKSZ
at this time. this is so as not to preclude applications from using
extremely small thread stacks when they know they will not be handling
signals. unfortunately cancellation and multi-threaded set*id() use
signals as an implementation detail and therefore require a stack
large enough for a signal context, so applications which use extremely
small thread stacks may still need to avoid using these features.

fix signal.h breakage from moving stack_t to arch-specific bits

in the previous changes, I missed the fact that both the prototype of
the sigaltstack function and the definition of ucontext_t depend on
stack_t.

move signal.h definition of stack_t to arch-specific bits

it's different at least on mips. mips version will be fixed in a
separate commit to show the change.

add deprecated SIGIOT alias for SIGABRT

reportedly some programs (e.g. showkeys in the kbd package) use it.

fixup mcontext stuff to expost gregset_t/fpregset_t as appropriate

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 up leftover, incorrect NSIG definitions in arch-specific signal.h

add back NSIG, removed from powerpc in last commit, but for all archs

unlike the previous definition, NSIG/_NSIG is supposed to be one more
than the highest signal number. adding this will allow simplifying
libc-internal code that makes signal-related syscalls, which can be
done as a later step. some apps might use it too; while this usage is
questionable, it's at least not insane.

microblaze port

based on initial work by rdp, with heavy modifications. some features
including threads are untested because qemu app-level emulation seems
to be broken and I do not have a proper system image for testing.