RISC-V: Improve the version parsing for arch string.

Keep the riscv_add_subset to do the same thing, and use a new
function, riscv_parse_add_subset, to cover most of the things
when parsing, including find the default versions for extensions,
and check whether the versions are valid.  The version 0p0 should
be an invalid version, that is the mistake I made before.  This
patch clarify the version rules as follows,

* We accept any version of extensions set by users, except 0p0.
* The non-standard x extensions must be set with versions in arch string.
* If user don't set the versions, or set 0p0 for the extensions, then try
  to find the supported versions according to the chosen ISA spec.
  Otherwise, report errors rather than output 0p0 for them.

Besides, we use as_bad rather than as_fatal to report more errors
for assembler.

	bfd/
	* elfxx-riscv.c (riscv_lookup_subset): Moved to front.
	(riscv_add_subset): Likewise.
	(riscv_release_subset_list): Likewise.
	(riscv_parse_add_subset): New function.  Find and check the
	versions before adding them by riscv_add_subset.
	(riscv_parsing_subset_version): Remove use_default_version
	and change the version type from unsigned to int.  Set the
	versions to RISCV_UNKNOWN_VERSION if we can not find them
	in the arch string.
	(riscv_parse_std_ext): Updated.
	(riscv_parse_prefixed_ext): Updated.  Since we use as_bad
	rather than as_fatal to report more errors, return NULL
	string if the parsed end_of_version is NULL, too.
	(riscv_parse_subset): Use a new boolean, no_conflict, to
	report more errors when we have more than one ISA conflicts.

	* elfxx-riscv.h (RISCV_DONT_CARE_VERSION): Changed to
	RISCV_UNKNOWN_VERSION.
	(riscv_lookup_subset_version): Removed.
	(riscv_parse_subset_t): Updated.

	gas/
	* config/tc-riscv.c (riscv_get_default_ext_version):
	Change the version type from unsigned to int.
	(riscv_set_arch): Use as_bad rather than as_fatal to
	report more errors.

	* testsuite/gas/riscv/attribute-02.d: Updated since x must be
	set with versions.
	* testsuite/gas/riscv/attribute-03.d: Likewise.
	* testsuite/gas/riscv/march-ok-two-nse.d: Likewise.
	* testsuite/gas/riscv/attribute-09.d: zicsr wasn't supported
	in the spec 2.2, so choose the newer spec.
	* testsuite/gas/riscv/march-fail-base-01.l: Updated since as_bad.
	* testsuite/gas/riscv/march-fail-base-02.l: Likewise.
	* testsuite/gas/riscv/march-fail-order-std.l: Likewise.
	* testsuite/gas/riscv/march-fail-order-x.l: Likewise.
	* testsuite/gas/riscv/march-fail-order-z.l: Likewise.
	* testsuite/gas/riscv/march-fail-porder.l: Likewise.
	* testsuite/gas/riscv/march-fail-rv32ef.l: Likewise.
	* testsuite/gas/riscv/march-fail-rv32id.l: Likewise.
	* testsuite/gas/riscv/march-fail-rv32iq.l: Likewise.
	* testsuite/gas/riscv/march-fail-rv64iq.l: Likewise.
	* testsuite/gas/riscv/march-fail-single-char.l: Likewise.
	* testsuite/gas/riscv/march-fail-unknown-std.l: Likewise.
	* testsuite/gas/riscv/march-fail-unknown.l: Likewise.
	* testsuite/gas/riscv/march-fail-uppercase.l: Likewise.
	* testsuite/gas/riscv/march-fail-version.l: Likewise.
	* testsuite/gas/riscv/march-fail-isa-spec.d: Likewise.
	* testsuite/gas/riscv/march-fail-isa-spec.l: Likewise.

	include/
	* opcode/riscv.h (riscv_ext_version):
	Change the version type from unsigned to int.

include/opcode/riscv.h

@@ -360,8 +360,8 @@ struct riscv_ext_version
 {
   const char *name;
   enum riscv_isa_spec_class isa_spec_class;
-  unsigned int major_version;
-  unsigned int minor_version;
+  int major_version;
+  int minor_version;
 };
 
 /* All RISC-V CSR belong to one of these classes.  */