58e291cfa Add test for GCC bug 90348 (Pieter Wuille)
Pull request description:
This adds a test for GCC bug 90348 (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90348), using a test case extracted from our own `sha256d64` test in crypto_tests.cpp, which was failing on some platforms.
This is based on top of #15983 to make sure the bug doesn't trigger (it does in some Travis configurations without it).
ACKs for commit 58e291:
Tree-SHA512: 4dc9084e92dd143a53930e42bb68e33d922a2a2b891406b259d3a0bed4511dcc49e7447a7a8e4eb793a26e3eacb188ca293b71e0e061f9b3230f8e7fcfd29525