RISC-V: Allow multi-lib build with different code model

RISC-V: Allow multi-lib build with different code model

--with-multilib-generator was only support for different ISA/ABI
combination, however code model is effect the code gen a lots it
should able to handled in multilib mechanism.

Adding `--cmodel=` option to `--with-multilib-generator` to generating
multilib combination with different code model.

E.g.
--with-multilib-generator="rv64ima-lp64--;--cmodel=medlow,medany"
will generate 3 multi-lib suppport:
1) rv64ima with lp64
2) rv64ima with lp64 and medlow code model
3) rv64ima with lp64 and medany code model

gcc/

	* config/riscv/multilib-generator: Support code model option for
	multi-lib.
	* doc/install.texi: Add document of new option for
	--with-multilib-generator.

(cherry picked from commit fdd40498d1981fde0720a0886d6f59ea5fb7ab40)