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.