Allow experimental ability to bootstrap go toolchain from source (#4553)

This allow users to apply targeted patches on top of the Go compiler in
an easy way. The motivating use case is
https://github.com/cerisier/toolchains_llvm_bootstrapped requiring a
small compiler patch that landed upstream but did not make the Go 1.26
release, but it can also be used to easily test Go compiler changes
against a Bazel codebase

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Co-authored-by: Son Luong Ngoc <sluongng@gmail.com>
Co-authored-by: Fabian Meumertzheim <fabian@meumertzhe.im>