gcc: re-organize multilib hierarchy for newlib gcc.
gcc: re-organize multilib hierarchy for newlib gcc.

Fix 1st-level hierarchy to 64/32 not soft-float. It is same with glibc gcc, also.

manuelafm/riscv-gnu-toolchain,manuelafm/riscv-gnu-toolchain,manuelafm/riscv-gnu-toolchain