RISC-V: Change -march parsing.

	bfd/
	2020-01-22  Maxim Blinov  <maxim.blinov@embecosm.com>
	* bfd/elfnn-riscv.c (riscv_skip_prefix): New.
	(riscv_prefix_cmp): Likewise.
	(riscv_non_std_ext_p): Deleted.
	(riscv_std_sv_ext_p): Likewise.
	(riscv_non_std_sv_ext_p): Likewise.
	(riscv_merge_non_std_and_sv_ext): Rename to...
	(riscv_merge_multi_letter_ext): and modified to use riscv_prefix_cmp.
	(riscv_merge_arch_attr_info): Replace 3 calls to
	riscv_merge_non_std_and_sv_ext with single call to
	riscv_merge_multi_letter_ext.
	* bfd/elfxx-riscv.c (riscv_parse_std_ext): Break if we
	encounter a 'z' prefix.
	(riscv_get_prefix_class): New function, return prefix class based
	on first few characters of input string.
	(riscv_parse_config): New structure to factor out minor differences
	in extension class parsing behaviour.
	(riscv_parse_sv_or_non_std_ext): Rename to...
	(riscv_parse_prefixed_ext): and parameterise with
	riscv_parse_config.
	(riscv_std_z_ext_strtab, riscv_std_s_ext_strtab): New.
	(riscv_multi_letter_ext_valid_p): New.
	(riscv_ext_x_valid_p, riscv_ext_z_valid_p, riscv_ext_s_valid_p): New.
	(riscv_parse_subset): Delegate all non-single-letter parsing work
	to riscv_parse_prefixed_ext.
	* bfd/elfxx-riscv.h (riscv_isa_ext_class): New type.
	(riscv_get_prefix_class): Declare.

	gas/
	2020-01-22  Maxim Blinov  <maxim.blinov@embecosm.com>
	* testsuite/gas/riscv/march-ok-s.d: sx is no longer valid and
	s exts must be known, so rename *ok* to *fail*.
	* testsuite/gas/riscv/march-ok-sx.d: Likewise.
	* testsuite/gas/riscv/march-ok-s-with-version: Likewise.
	* testsuite/gas/riscv/march-fail-s.l: Expected error messages for
	above change.
	* testsuite/gas/riscv/march-fail-sx.l: Likewise.
	* testsuite/gas/riscv/march-fail-sx-with-version.l: Likewise.

Change-Id: Ic4d91a13d055a10d30ab28752a380a669b59f29c