RISC-V: Minor updates for architecture parser.

* Two add subset functions is redundant.  Keep the riscv_add_implicit_subset,
and renamed it to riscv_add_subset.  Besides, if the subset is added in order,
then we just add it at the tail of the subset list.

* Removed the "-march:" prefix from the error messages.  Since not only the
-march= option will use the parser, but also the architecture elf attributes,
the default architecture setting and linker will use the same parser.

* Use a function, riscv_parse_check_conflicts, to check the conflicts
of extensions, including the rv64e and rv32q.

The rv32emc-elf/rv32i-elf/rv32gc-linux/rv64gc-elf/rv64gc-linux regressions
are tested and passed.

bfd/
	* elfxx-riscv.c (riscv_lookup_subset): Check the subset tail list
	first.  If the subset is added in order, then we can just add it to
	the tail without searching the whole list.
	(riscv_add_subset): Replaced by riscv_add_implicit_subset.
	(riscv_add_implicit_subset): Renamed to riscv_add_subset.
	(riscv_parse_add_subset): Updated.
	(riscv_parsing_subset_version): Removed the "-march:" prefix from
	the error message.
	(riscv_parse_prefixed_ext): Likewise.
	(riscv_parse_std_ext): Likewise.  And move the rv<xlen>e check
	to riscv_parse_check_conflicts.
	(riscv_parse_check_conflicts): New function used to check conflicts.
	(riscv_parse_subset): Updated.
gas/
	* testsuite/gas/riscv/march-fail-base-02.l: Updated.
	* testsuite/gas/riscv/march-fail-unknown-std.l: Likewise.