include: de-macro __ssp_overlap(), improve semantics and checking

[kevans]

Summary

Switch away from pointer arithmetic to provide more obvious semantics
for checking overlap on pointer ranges. This lets us remove some casts
that need not exist and removes some possible fragility in its use.

While we're here, check for overflow just in case; sometimes we use a
caller-supplied size if __builtin_object_size(3) can't deduce the buffer
size, and we should fail the check if the size is nonsensical for the
provided buffers.

[markj]

include/ssp/ssp.h

101

The right <= left condition is redundant, at this point we must have right < left.

include/ssp/wchar.h

34

Extra ws after #define

35

Do we need to check that len * sizeof(wchar_t) doesn't overflow?

[kevans]

Address review commentary, remove redundant bits and ad an overflow check to
__ssp_wchar_overlap (and make it an inline function as well, instead of a macro).