Complement commit bfe78b08 ("RISC-V: Using fmv.x.w/fmv.w.x rather
than fmv.x.s/fmv.s.x") and document a binutils 2.30 requirement in the
installation manual, matching the addition of fmv.x.w/fmv.w.x mnemonics
to GAS.

	gcc/
	* doc/install.texi (Specific) <riscv32-*-elf, riscv32-*-linux>
	<riscv64-*-elf, riscv64-*-linux>: Update binutils requirement to
	2.30.