https://gitlab.com/freepascal.org/fpc/source/-/issues/39322

This suggests that this project1.bc file has been generated by a newer version of clang/llvm than the llvm version used by the linker's lto plugin (LLVMgold.so).
By default, the linker will use the plugin that's symlinked by /usr/lib/bfd-plugins/liblto_plugin.so. That one is probably symlinked to an older version of LLVM compared to the one you're using to create the bitcode. I don't know which package is normally expected to update that symlink, or how you can configure it besides manually changing it. Alternately, you may also be able to manually override the location of the lto plugin to use by passing "-plugin /usr/lib/llvm-10/lib/LLVMgold.so" to the linker (so with fpc options, that would be -k-plugin -k/usr/lib/llvm-10/lib/LLVMgold.so).