Merge #19959: build: patch qt libpng to fix powerpc build

f07fb5a55e2d5d87f288c988a24ae8b9e3f827a1 build: patch qt libpng to fix powerpc build (fanquake)

Pull request description:

  This is an alternative to #19751 that fixes the build without requiring
  splitting out libpng. This patch can be dropped once we are building qt
  5.12.0 or later.

  One of the [concerns posted in #19751](https://github.com/bitcoin/bitcoin/pull/19751#issuecomment-675455993) was:

  > the one bundled with Qt is at best a slower "bare minimum".

  However for our usage, I don't think the performance of libpng is a concern. If it is, I'd like to see some numbers/justification as to why we should be worried about it.

  This patch should be enough to unblock PowerPC binaries (combined with other changes) for 0.21.0, and for 0.22.0, when we switch to qt 5.15.x in depends, we should be able to drop it.

  Related upstream issue: https://bugreports.qt.io/browse/QTBUG-66388.

ACKs for top commit:
  laanwj:
    ACK f07fb5a55e2d5d87f288c988a24ae8b9e3f827a1
  theuni:
    ACK f07fb5a55e2d5d87f288c988a24ae8b9e3f827a1.
  hebasto:
    ACK f07fb5a55e2d5d87f288c988a24ae8b9e3f827a1, the patch is the same as https://codereview.qt-project.org/gitweb?p=qt/qtbase.git;a=blobdiff;f=src/3rdparty/libpng/libpng.pro;h=a2f56669b47e09629b7a88a0964091404d6a9b06;hp=577b61d833f7842f3f0d6b1c94a3b3920006695c;hb=dc2aead842f4cdf74f9259d3606c53c8bdae2c6b;hpb=ceeecbae510af6e2d1ebbf865761e4761d404033

Tree-SHA512: 865b843f5049eca80215774274fb7ae0dacccc3dd7f4a2eec93a73057115dcea85e715f919f96441424f9dd902dd97f0a238d96d4074babcee66b30577104009

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