sha256_shani.cpp + clang: suppress -Wcast-align warnings

Summary:

This warning occurs if you are using clang with an architecture that has AVX and SHA extensions (newer Intel CPUs). src/crypto/sha256_shani.cpp uses these special functions by casting various shorter types such as int to _m128i * and other wider pointers. On GCC it appears no warning is generated, but on clang you do get a warning.

Since we were fixing warnings, I decided to also fix this one which always irked me.

Test Plan:

• use clang as the C++ compiler
• be on a machine with Intel AVX (newer CPUs)
• ninja check or ninja check-secp256k1 or just plain ninja will do it. Before this commit, you should get warnings from src/crypto/sha256_shani.cpp and after this commit, none emitted from that file.

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This is the warning that is fixed: