Cleanup riscv option docs.

Cleanup riscv option docs.

	gcc/
	* doc/invoke.texi (RISC-V Options): Delete nonexistent -mmemcpy and
	-mno-memcpy options.  For -mplt, -mfdiv, -mdiv, -msave-restore, and
	-mstrict-align, add info on default value.  Delete redundant lines for
	-mabi.  Add missing -mexplicit-relocs docs.

From-SVN: r255280