Merge #13386: SHA256 implementations based on Intel SHA Extensions

66b2cf1ccfad545a8ec3f2a854e23f647322bf30 Use immintrin.h everywhere for intrinsics (Pieter Wuille)
4c935e2eee456ff66cdfb908b0edffdd1e8a6c04 Add SHA256 implementation using using Intel SHA intrinsics (Pieter Wuille)
268400d3188200c9e3dcd3482c4853354388a721 [Refactor] CPU feature detection logic for SHA256 (Pieter Wuille)

Pull request description:

  Based on #13191.

  This adds SHA256 implementations that use Intel's SHA Extension instructions (using intrinsics). This needs GCC 4.9 or Clang 3.4.

  In addition to #13191, two extra implementations are provided:
  * (a) A variable-length SHA256 implementation using SHA extensions.
  * (b) A 2-way 64-byte input double-SHA256 implementation using SHA extensions.

  Benchmarks for 9001-element Merkle tree root computation on an AMD Ryzen 1800X system:
  * Using generic C++ code (pre-#10821): 6.1ms
  * Using SSE4 (master, #10821): 4.6ms
  * Using 4-way SSE4 specialized for 64-byte inputs (#13191): 2.8ms
  * Using 8-way AVX2 specialized for 64-byte inputs (#13191): 2.1ms
  * Using 2-way SHA-NI specialized for 64-byte inputs (this PR): 0.56ms

  Benchmarks for 32-byte SHA256 on the same system:
  * Using SSE4 (master, #10821): 190ns
  * Using SHA-NI (this PR): 53ns

  Benchmarks for 1000000-byte SHA256 on the same system:
  * Using SSE4 (master, #10821): 2.5ms
  * Using SHA-NI (this PR): 0.51ms

Tree-SHA512: 2b319e33b22579f815d91f9daf7994a5e1e799c4f73c13e15070dd54ba71f3f6438ccf77ae9cbd1ce76f972d9cbeb5f0edfea3d86f101bbc1055db70e42743b7

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