RISC-V: Enable lui relaxation for CODE and MERGE sections.

	2019-06-24  Ilia Diachkov  <ilia.diachkov@optimitech.com>
	bfd/
	* elfnn-riscv.c (_bfd_riscv_relax_lui): Delete early exit when
	SEC_MERGE or SEC_CODE flags are set.
	(_bfd_riscv_relax_section): New local symtype.  Set sym_sec and
	symtype consistently.  Don't include sec_addr (sym_sec) in symval.
	Add check for SEC_INFO_TYPE_MERGE and call _bfd_merged_section_offset.
	Add sec_addr (sym_sec) after handling merge sections.