Merge #10821: Add SSE4 optimized SHA256

Merge #10821: Add SSE4 optimized SHA256

6b8d872 Protect SSE4 code behind a compile-time flag (Pieter Wuille)
fa9be90 Add selftest for SHA256 transform (Pieter Wuille)
c1ccb15 Add SSE4 based SHA256 (Pieter Wuille)
2991c91 Add SHA256 dispatcher (Pieter Wuille)
4d50f38 Support multi-block SHA256 transforms (Pieter Wuille)

Pull request description:

  This adds an SSE4 assembly version of the SHA256 transform by Intel, and uses it at run time if SSE4 instructions are available, and use a fallback C++ implementation otherwise. Nearly every x86_64 CPU supports SSE4. The feature is only enabled when compiled with `--enable-experimental-asm`.

  In order to avoid build dependencies and other complications, the original Intel YASM code was translated to GCC extended asm syntax.

  This gives around a 50% speedup on the SHA256 benchmark for me.

  It is based on an earlier patch by @laanwj, though only includes a single assembly version (for now), and removes the YASM dependency.

Tree-SHA512: d31c50695ceb45264291537b93c0d7497670be38edf021ca5402eaa7d4e1e0e1ae492326e28d4e93979d066168129e62d1825e0384b1b906d36f85d93dfcb43c