Hello,
64bit builds are not supported at this time.
Stefan
As of now, coreboot on x86 is primarily 32bit. This needs a 32bit toolchain to properly build. Back when the coreboot toolchain was GCC6, the x64 toolchain was built with a multilib patch, which means it could build 32bit and 64bit binaries. However, when upgrading to GCC8, that multilib patch was not updated (thus removed), hence the error you get.
Regards,
Angel Pons