RISC-V: Add --with-cmodel configure option

RISC-V: Add --with-cmodel configure option

Sometimes we want to use default cmodel other than medlow. Add a GCC
configure option for that.

gcc/ChangeLog:

	* config.gcc (riscv*-*-*): Add support for --with-cmodel configure option.
	(all_defaults): Add cmodel.
	* config/riscv/riscv.h (TARGET_DEFAULT_CMODEL): Remove.
	* doc/install.texi: Document --with-cmodel configure option.
	* doc/invoke.texi (-mcmodel): Mention --with-cmodel configure option.

Co-authored-by: Kito Cheng <kito.cheng@sifive.com>