RISC-V: Support --with-cpu

RISC-V: Support --with-cpu

The --with-cpu will allow riscv compilers to have a default mcpu flag.
Setting -mcpu or -march at compile time will override the configured
--with-cpu.

gcc/ChangeLog:

	* config.gcc: Add cpu to supported configure options
	* config/riscv/riscv.h (riscv_arch_help): Use --with-cpu during
	compilation
	* doc/install.texi: Mention in docs that --with-cpu is supported