RISC-V Multilib

As riscv has a few ABIs multilib story it a bit complicated as well.

Gentoo will use multilib compatible LIBDIR layout for both multilib and non-multilib profiles. TODO: add details on gcc's multilib LIBDIR and compiler flags setup and on what other distros do.

rv64gc
This is the 64bit architecture (rv64) with extensions imadfc (i.e., g is a shorthand for imadf). It supports 2 ABIs, lp64 (softfloat) and lp64d (hardfloat double).

The gcc default is -march=rv64gc -mabi=lp64d

Our multilib structure follows the guidelines as implemented in GCC: LIBDIR_lp64d="lib64/lp64d" LIBDIR_lp64="lib64/lp64" SYMLINK_LIB="no" CFLAGS_lp64d="-mabi=lp64d" CFLAGS_lp64="-mabi=lp64"
 * 1) from profiles/arch/riscv/rv64gc/make.defaults
 * 1) Library directories
 * 1) Flags for lp64d
 * 1) Flags for lp64

rv32i
This is the 32bit architecture, which is not supported by glibc yet.