RISC-V: Enable riscv attributes by default for all riscv targets.

RISC-V: Enable riscv attributes by default for all riscv targets.

These were only enabled for embedded elf originally because that was
the safe option, and linux had no obvious use for them.  But now that
we have new extensions coming like V that affect process state and ABIs,
the attributes are expected to be useful for linux, and may be required
by the psABI.  clang already emits them for all riscv targets.

	gcc/
	* config.gcc (riscv*-*-*): If --with-riscv-attribute not used,
	turn it on for all riscv targets.