this is actually a functional fix at present, since the C sqrtl does
not support ld80 and just wraps double sqrt. once that's fixed it will
just be an optimization.

if len==0, an uninitalized variable would be returned

the previous commit addressing async-signal-safety issues around
pthread_kill did not fully fix pthread_cancel, which is also required
(albeit rather irrationally) to be async-cancel-safe.

without blocking implementation-internal signals, it's possible that,
when async cancellation is enabled, a cancel signal sent by another
thread interrupts pthread_kill while the killlock for a targeted
thread is held. as a result, the calling thread will terminate due to
cancellation without ever unlocking the targeted thread's killlock,
and thus the targeted thread will be unable to exit.

pthread_kill is required to be AS-safe. that requirement can't be met
if the target thread's killlock can be taken in contexts where
application-installed signal handlers can run.

block signals around use of this lock in all pthread_* functions which
target a tid, and reorder blocking/unblocking of signals in
pthread_exit so that they're blocked whenever the killlock is held.

this broke mallocng size_to_class on archs without a native
implementation of a_clz_32. the incorrect logic seems to have been
something i derived from a related but distinct log2-type operation.
with the change made here, it passes an exhaustive test.

as this function is new and presently only used by mallocng, no other
functionality was affected.

vfscanf() may use the variable 'alloc' uninitialized when taking the
branch introduced by commit b287cd745c2243f8e5114331763a5a9813b5f6ee.
Spotted by clang.

the files added come from the mallocng development repo, commit
2ed58817cca5bc055974e5a0e43c280d106e696b. they comprise a new malloc
implementation, developed over the past 9 months, to replace the old
allocator (since dubbed "oldmalloc") with one that retains low code
size and minimal baseline memory overhead while avoiding fundamental
flaws in oldmalloc and making significant enhancements. these include
highly controlled fragmentation, fine-grained ability to return memory
to the system when freed, and strong hardening against dynamic memory
usage errors by the caller.

internally, mallocng derives most of these properties from tightly
structuring memory, creating space for allocations as uniform-sized
slots within individually mmapped (and individually freeable)
allocation groups. smaller-than-pagesize groups are created within
slots of larger ones. minimal group size is very small, and larger
sizes (in geometric progression) only come into play when usage is
high.

all data necessary for maintaining consistency of the allocator state
is tracked in out-of-band metadata, reachable via a validated path
from minimal in-band metadata. all pointers passed (to free, etc.) are
validated before any stores to memory take place. early reuse of freed
slots is avoided via approximate LRU order of freed slots. further
hardening against use-after-free and double-free, even in the case
where the freed slot has been reused, is made by cycling the offset
within the slot at which the allocation is placed; this is possible
whenever the slot size is larger than the requested allocation.

this includes both an implementation of reclaimed-gap donation from
ldso and a version of mallocng's glue.h with namespace-safe linkage to
underlying syscalls, integration with AT_RANDOM initialization, and
internal locking that's optimized out when the process is
single-threaded.

these are based on the ARM optimized-routines repository v20.05
(ef907c7a799a), with macro dependencies flattened out and memmove code
removed from memcpy. this change is somewhat unfortunate since having
the branch for memmove support in the large n case of memcpy is the
performance-optimal and size-optimal way to do both, but it makes
memcpy alone (static-linked) about 40% larger and suggests a policy
that use of memcpy as memmove is supported.

tabs used for alignment have also been replaced with spaces.

Allow the existing ARM assembler memcpy implementation to be used for
both big and little endian targets.