crypto/sha256: Use pragmas to enforce necessary intrinsics for GCC and Clang #13789
This avoids problems when the user specifies CXXFLAGS explicitly disabling the relevant optimisations.
Partially fixes #13758, only for GCC/Clang.
Concept ACK, I think this is cleaner than doing it in the build system, actually. Maybe it makes sense to add a macro in a util/ file so no compiler-specific code is needed here.
sha256_sse4.cpp does not need this, as it's written using inline assembly, not intrinsics.