RISC-V: Gas inserts cfa relocs in wrong section.

RISC-V: Gas inserts cfa relocs in wrong section.

The frag code makes a distinction between inserting frags before the frag
chains are chained together and afterward.  After chaining, we need to set
now_seg before creating the frag.  tc-xtensa.c has a function called
fix_new_exp_in_seg that handles this right, but switches segments twice each
time it is called.  In this case, we can inline it and pull the save and
restore out of the loop to get better code.

	gas/
	PR 26025
	* config/tc-riscv.c (riscv_pre_output_hook): Change s type from const
	asection to segT.  New locals seg and subseg.  Call subseg_set before
	fix_new_exp.  Call subseg_set after loop to restore original values.