RISC-V/doc: Add index references for mrelax' and mriscv-attribute'

Add missing index references for the `-mrelax' and `-mriscv-attribute'
invocation options.

	gcc/
	* doc/invoke.texi (RISC-V Options): Add index references for
	`mrelax' and `mriscv-attribute'.

gcc/doc/invoke.texi

@@ -28373,12 +28373,14 @@ limit optimization.
 
 @item -mrelax
 @itemx -mno-relax
+@opindex mrelax
 Take advantage of linker relaxations to reduce the number of instructions
 required to materialize symbol addresses. The default is to take advantage of
 linker relaxations.
 
 @item -mriscv-attribute
 @itemx -mno-riscv-attribute
+@opindex mriscv-attribute
 Emit (do not emit) RISC-V attribute to record extra information into ELF
 objects.  This feature requires at least binutils 2.32.