This will fix the crypto_tests failure on 32-bits platforms.

Backport of core PR15983:
https://github.com/bitcoin/bitcoin/pull/15983/files

With CMake the flag is set for C++ as well as C, while autotools only
has the C++ flag. This is on purpose as the bug is also affecting gcc
and not only g++. I ran the benchmarks and this caused no noticeable
runtime difference.

Fun fact: you can also get rid of the bug by disabling the optimizations
(i.e. build with debug enabled).

cmake -GNinja .. \
  -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/Linux32.cmake \
  -DENABLE_STATIC_LIBSTDCXX=ON \
  -DENABLE_GLIBC_BACK_COMPAT=ON
ninja test_bitcoin
./src/test/test_bitcoin -t crypto_tests

Make sure the test is green. Note that this can't run on WSL because the
kernel only supports 64-bits binaries.