RISC-V: Add custom RTEMS multilibs

RISC-V: Add custom RTEMS multilibs

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.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@261619 138bc75d-0d04-0410-961f-82ee72b054a4