RISC-V: Support B, Zaamo and Zalrsc extensions.

RISC-V: Support B, Zaamo and Zalrsc extensions.

* https://github.com/riscv/riscv-b/tags
Added standard B extension back, which implies Zba, Zbb and Zbs extensions.

* https://github.com/riscv/riscv-zaamo-zalrsc/tags
Splited standard A extension into two new extensions, Zaamo and Zalrsc.
The A extension implies Zaamo and Zalrsc extensions.

Not sure if we need to do the similar check as i and zicsr/zifencei.

Passed riscv[32|64]-[elf/linux] binutils testcases.

bfd/
	* elfxx-riscv.c (riscv_implicit_subsets): Added imply rules
	for A and B extensions.  The A implies Zaamo and Zalrsc, the
	B implies Zba, Zbb and Zbs.
	(riscv_supported_std_ext): Supported B extension with v1.0.
	(riscv_supported_std_z_ext): Supported Zaamo and Zalrsc with v1.0.
	(riscv_multi_subset_supports, riscv_multi_subset_supports_ext): Updated.
include/
	* opcode/riscv.h (riscv_insn_class): Removed INSN_CLASS_A, Added
	INSN_CLASS_ZAAMO and INSN_CLASS_ZALRSC.
opcodes/
	* riscv-opc.c (riscv_opcodes): Splited standard A extension into two
	new extensions, Zaamo and Zalrsc.
gas/
	* testsuite/gas/riscv/march-imply-a.d: New testcase.
	* testsuite/gas/riscv/march-imply-b.d: New testcase.
	* testsuite/gas/riscv/attribute-01.d: Updated.
	* testsuite/gas/riscv/attribute-02.d: Updated.
	* testsuite/gas/riscv/attribute-03.d: Updated.
	* testsuite/gas/riscv/attribute-04.d: Updated.
	* testsuite/gas/riscv/attribute-05.d: Updated.
	* testsuite/gas/riscv/attribute-10.d: Updated.
	* testsuite/gas/riscv/mapping-symbols.d: Updated.
	* testsuite/gas/riscv/march-imply-g.d: Updated.
	* testsuite/gas/riscv/march-imply-unsupported.d: Updated.
	* testsuite/gas/riscv/march-ok-reorder.d: Updated.
ld/
	* testsuite/ld-riscv-elf/attr-merge-arch-01.d: Updated.
	* testsuite/ld-riscv-elf/attr-merge-arch-02.d: Updated.
	* testsuite/ld-riscv-elf/attr-merge-arch-03.d: Updated.
	* testsuite/ld-riscv-elf/attr-merge-user-ext-01.d: Updated.