Add CRC intrinsics when __CRC__ is not defined

[noloader]

This issue tracks adding CRC intrinsics when __CRC__ is not defined. This happens when a machine lacks CRC or -march=native is not used. While it may seem counter-intuitive to add it when its not needed, the primary use case is distro's, which builds for generic x86_64 and its users who have a more capable machine. A secondary use case is Clang, which does not enable all cpu features under -march=native on occasion.

Also see Fixing "ERROR: failed to generate sha1rnds4 instruction" (and friends) on the mailing list.

[noloader]

Cleared at Pull Request 461.

PR 461 takes the "single source" and breaks it into a "base implementation" which is standard C++, and a "SIMD implementation" which includes different ISAs.

The base implementation can be built with minimal or no flags. For example, -march=i686 or -march=x86-64. The SIMD implementation will be built with architecture specific flags, like -msha or -march=armv8-a+crypto when compiling sha-simd.cpp.

The GNUmakefile will add the flags automatically. Other build systems, like Cmake may need to be modified. Its OK to specify -march=native, and in fact the makefile still uses this strategy unless the user disables it. However, the makefile always adds the architecture specific flag to a source file when needed to ensure a source file compiles correctly.