analogous to the bug in wcscmp and wcsncmp that was fixed in commit
07616721f1fa6cb215ffbef23441cae80412484f.

As a result of using simple subtraction to implement the return values
for wcscmp and wcsncmp, integer overflow can occur (producing
undefined behavior, and in practice, a wrong comparison result). This
does not occur for meaningful character values (21-bit range) but the
functions are specified to work on arbitrary wchar_t arrays.

This patch replaces the subtraction with a little bit of code that
orders the characters correctly, returning -1 if the character from
the first string is smaller than the one from the second, 0 if they
are equal and 1 if the character from the first string is larger than
the one from the second.

the rule that longest digit sequence not beginning with a zero is
greater only applies when both sequences being compared are
non-degenerate. this is spelled out explicitly in the man page, which
may be deemed authoritative for this nonstandard function: "If one or
both of these is empty, then return what strcmp(3) would have
returned..."

we were wrongly treating any sequence of digits not beginning with a
zero as greater than a non-digit in the other string.

the intent here is just to scan at least l bytes forward for the end
of the haystack and at least some decent minimum to avoid doing it
over and over if the needle is short, with no need to be precise. the
comment erroneously stated this as an estimate for MIN when it's
actually an estimate for MAX.

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.

presently all archs define SIGSTKFLT but this is not correct. change
strsignal as a prerequisite for fixing that.

unsigned char promotes to int, which can overflow when shifted left by
24 bits or more. this has been reported multiple times but then
forgotten. it's expected to be benign UB, but can trap when built with
explicit overflow catching (ubsan or similar). fix it now.

note that promotion to uint32_t is safe and portable even outside of
the assumptions usually made in musl, since either uint32_t has rank
at least unsigned int, so that no further default promotions happen,
or int is wide enough that the shift can't overflow. this is a
desirable property to have in case someone wants to reuse the code
elsewhere.

Commit d9bdfd164 ("fix memccpy to not access buffer past given size")
correctly added a check for 'n' nonzero, but made the pre-existing test
'*s==c' redundant: n!=0 implies *s==c. Remove the unnecessary check.

Reported by Alexey Izbyshev.

For Thumb2 compatibility, replace two instances of a single
instruction "orr with a variable shift" with the two instruction
equivalent. Neither of the replacements are in a performance critical
loop.