Add multilib variants for -march=rv64imafd, e.g. to support the BOOMv2 core.

Add -mcmodel=medany as a variant of the 64-bit multilibs for RTEMS.  The
rationale for this change is that several existing RISC-V chips map the
RAM at 0x80000000.  In RTEMS, we do not use virtual memory, so
applications will run at this location which is outside the +-2GiB range
in a 64-bit configuration.

gcc/
	* config.gcc (riscv*-*-elf* | riscv*-*-rtems*): Use custom
	multilibs for *-*-rtems*.
	* config/riscv/t-rtems: New file.

From-SVN: r261619