The lp64d stages can be used on all 64-bit RISC-V processors supporting the double-precision floating point instruction set. The lp64 stages can be used on all 64-bit RISC-V processors; they require no hardware support for floating point arithmetics.
For an overview of the different instruction sets, see, e.g., the RISC-V wikipedia page.
Additional Gentoo-specific information can be found on the web page of the Gentoo RISC-V project.