RISC-V: Fix the behavior for multilib-generator with --cmodel=large on rv32

RISC-V: Fix the behavior for multilib-generator with --cmodel=large on rv32

Large code model is only supported on RV64, so we don't need to
generate the multilibs for RV32 with --cmodel=large. And the compact
code model is something we don't supported on upstream (which is
accidentally added in the past), so we need to remove it.

gcc/ChangeLog:

	* config/riscv/multilib-generator: Remove the compact code model
	and check large code model for RV32.

(cherry picked from commit 72dff34bcdd6f05b64bbf07739ab815e673b5946)