RISC-V: Minor cleanup and testcases improvement for arch string parser.

RISC-V: Minor cleanup and testcases improvement for arch string parser.

Re-indent the related codes, unify and improve the related error messages
and comments.  Besies, also re-write the testcases to cover more cases.

	bfd/
	* elfxx-riscv.c: Re-indent codes, unify and improve the error
	messages and comments.
	(riscv_parse_prefixed_ext): Stop parsing the prefixed class
	extensions if the class is RV_ISA_CLASS_UNKNOWN, I get internal
	errors before adding this check for march-fail-porder* testcases.
	(riscv_parse_subset): Move the rv32 with q checking in front.
	* elfxx-riscv.h: Likewise.

	gas/
	(These are new testcases that cover more cases)
	* testsuite/gas/riscv/march-fail-base-01.d: The first extension must
	be e, i or g.
	* testsuite/gas/riscv/march-fail-base-01.l: Likewise.
	* testsuite/gas/riscv/march-fail-base-02.d: rv64e is an invalid base ISA.
	* testsuite/gas/riscv/march-fail-base-02.l: Likewise.
	* testsuite/gas/riscv/march-fail-order-std.d: Check orders of standard
	extensions.
	* testsuite/gas/riscv/march-fail-order-std.l: Likewise.
	* testsuite/gas/riscv/march-fail-order-x.d: Check orders of prefixed
	x extensions.
	* testsuite/gas/riscv/march-fail-order-x.l: Likewise.
	* testsuite/gas/riscv/march-fail-porder-x-std.d: Check orders when
	standard and prefixed extensions are set at the same time.
	* testsuite/gas/riscv/march-fail-porder-x-z.d: Likewise.
	* testsuite/gas/riscv/march-fail-porder-z-std.d: Likewise.
	* testsuite/gas/riscv/march-fail-porder.l: Likewise.
	* testsuite/gas/riscv/march-fail-single-char-s.d: Only standard
	extensions can use single char.
	* testsuite/gas/riscv/march-fail-single-char-x.d: Likewise.
	* testsuite/gas/riscv/march-fail-single-char-z.d: Likewise.
	* testsuite/gas/riscv/march-fail-single-char.l: Likewise.
	* testsuite/gas/riscv/march-fail-unknown-s.d: All extensions
	should be known, except the non-standard x extensions.
	* testsuite/gas/riscv/march-fail-unknown-std.d: Likewise.
	* testsuite/gas/riscv/march-fail-unknown-std.l: Likewise.
	* testsuite/gas/riscv/march-fail-unknown-z.d: Likewise.
	* testsuite/gas/riscv/march-fail-unknown.l: Likewise.
	* testsuite/gas/riscv/march-fail-uppercase-base.d: Do not
	allow any uppercase in the arch string.
	* testsuite/gas/riscv/march-fail-uppercase-std.d: Likewise.
	* testsuite/gas/riscv/march-fail-uppercase-z.d: Likewise.
	* testsuite/gas/riscv/march-fail-uppercase.l: Likewise.
	* testsuite/gas/riscv/march-fail-version-x.d: Failed to set versions.
	* testsuite/gas/riscv/march-fail-version-z.d: Likewise.
	* testsuite/gas/riscv/march-fail-version.l: Likewise.
	* testsuite/gas/riscv/march-fail-rv32ef.l: Updated.
	* testsuite/gas/riscv/march-fail-rv32id.d: Need f-ext.
	* testsuite/gas/riscv/march-fail-rv32iq.d: Should be rv64.
	* testsuite/gas/riscv/march-fail-rv32iq.l: Likewise.
	* testsuite/gas/riscv/march-fail-rv64iq.d: Need d-ext and f-ext.
	* testsuite/gas/riscv/march-fail-rv64iq.l: Likewise.

	(The following testcases are removed and covered by new testcases)
	* testsuite/gas/riscv/march-fail-rv32i.d: march-fail-uppercase-base.
	* testsuite/gas/riscv/march-fail-rv32i.l: Likewise.
	* testsuite/gas/riscv/march-fail-rv32iam.d: march-fail-order-std.
	* testsuite/gas/riscv/march-fail-rv32iam.l: Likewise.
	* testsuite/gas/riscv/march-fail-rv32ic.d: march-fail-uppercase-std.
	* testsuite/gas/riscv/march-fail-rv32ic.l: Likewise.
	* testsuite/gas/riscv/march-fail-rv32icx2p.d: march-fail-version-x.
	* testsuite/gas/riscv/march-fail-rv32icx2p.l: Likewise.
	* testsuite/gas/riscv/march-fail-rv32imc.d: march-fail-order-std.
	* testsuite/gas/riscv/march-fail-rv32imc.l: Likewise.
	* testsuite/gas/riscv/march-fail-rv64I.d: march-fail-uppercase-std.
	* testsuite/gas/riscv/march-fail-rv64I.l: Likewise.
	* testsuite/gas/riscv/march-fail-rv64e.d: march-fail-base-02.
	* testsuite/gas/riscv/march-fail-rv64e.l: Likewise.
	* testsuite/gas/riscv/march-fail-s-with-version.d: march-fail-unknown-s.
	* testsuite/gas/riscv/march-fail-s-with-version.l: Likewise.
	* testsuite/gas/riscv/march-fail-s.d: march-fail-unknown-s.
	* testsuite/gas/riscv/march-fail-s.l: Likewise.
	* testsuite/gas/riscv/march-fail-sx.d: march-fail-unknown-s.
	* testsuite/gas/riscv/march-fail-sx.l: Likewise.