Constants for host and target triples

[HertzDevil]

Crystal::DESCRIPTION can be mostly recreated from the other constants under Crystal:

formatted_sha = "[#{Crystal::BUILD_COMMIT}] " if Crystal::BUILD_COMMIT
<<-EOS
Crystal #{Crystal::VERSION} #{formatted_sha}(#{Crystal::BUILD_DATE})

LLVM: #{Crystal::LLVM_VERSION}
Default target: x86_64-pc-linux-gnu
EOS

This is almost a 1:1 replica of the internal Crystal::Config.description, except that the default LLVM target triple must still be hardcoded. It probably doesn't hurt to expose it:

Crystal::HOST_TRIPLE   # => "x86_64-pc-linux-gnu"
Crystal::TARGET_TRIPLE # => "x86_64-pc-linux-gnu"

Then Crystal::HOST_TRIPLE can be used in the above interpolation. TARGET_TRIPLE would be equal to whatever is passed to the --target command-line option, possibly normalized, and defaults to HOST_TRIPLE if not given.

Slightly related: #11694