RISC-V: Reduce effective linker relaxation passses

Commit 43025f01 ("RISC-V: Improve link time complexity.") reduced the
time complexity of the linker relaxation but some code portions did not
reflect this change.

This commit fixes a comment describing each relaxation pass and reduces
actual number of passes for the RISC-V linker relaxation from 3 to 2.
Though it does not change the functionality, it marginally improves the
performance while linking large programs (with many relocations).

bfd/ChangeLog:

	* elfnn-riscv.c (_bfd_riscv_relax_section): Fix a comment to
	reflect current roles of each relaxation pass.

ld/ChangeLog:

	* emultempl/riscvelf.em: Reduce the number of linker relaxation
	passes from 3 to 2.