This commit adds `gcc/common/checksum.nix`, which contains code
common to both gcc11 and gcc12, implementing the `enableChecksum`
feature.
When gcc's built-in bootstrap (`--enable-bootstrap`) is used, gcc
compiles itself three times and compares a hash of the unlinked `.o`
files from the second and third compilation. The
`enableChecksum=true` parameter performs the same comparison as part
of the `postInstall` phase.
Notably, `enableChecksum=true` can be used with `enableBootstrap=false`.
Co-authored-by: Sandro <sandro.jaeckel@gmail.com>
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