Makefile: Fix command line limits for static libs

I have been hitting the Windows command-line limit when cross-compiling
static library (POCO) and having more than 8000 characters in the call
to "ar". Calculating exact limits here are tricky, since this particular
limit will only take into account object file strings, which is correct
for response files, but not for the archive rules (link.txt files),
since they also contain the call to "ar" and its arguments.
Also, there can be other additional arguments if "ar" tool is wrapped
into something else, so it is a good idea to leave more space than
trying to exactly fit the limit.
Since response files use half of the limit as a heuristic, we reproduce
it here for consistency.

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