boost: Update to 1_76_0 and using python3

boost: Update to 1_76_0 and using python3

- Update from 1_71_0 to 1_76_0
- Update rootfile for x86_64 and copy for other architectures by replacing
    x64 with x32 for i586, a32 for armv5tel and a64 for aarch64
- Make build use python3
- add link=shared to build to only have shared libraries created, except for
    libboost_exception and libboost_test_exec_monitor which are only
    created as static versions
- Changelog
    Fixes
      algorithm::reduce with crop now does not remove the counts in flow
        bins anymore if the selected range actually overlaps with the flow
        bins, making the treatment of flow bins consistent with inner bins
      accumulators::mean and accumulators::weighted_mean now compute the
        correct variance when operator+= was called, for example, when
        histograms with accumulators are added; this was wrong before
        leading to too small variances
      detection of weight support in user-defined accumulators was broken
        at compile-time if accumulator used operator+= instead of
        operator(); tests only accidentally passed for builtin weighted_sum

Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>