RISC-V: Add support for Zbs instructions

RISC-V: Add support for Zbs instructions

This change adds the Zbs instructions from the Zbs 1.0.0 specification.
See
  https://github.com/riscv/riscv-bitmanip/releases/tag/1.0.0
for the frozen specification.

2021-01-09  Philipp Tomsich  <philipp.tomsich@vrull.eu>

    bfd/
	* elfxx-riscv.c (riscv_supported_std_z_ext): Added zbs.
    gas/
	* config/tc-riscv.c (riscv_multi_subset_supports): Handle INSN_CLASS_ZBS.
	* testsuite/gas/riscv/b-ext.d: Test Zbs instructions.
	* testsuite/gas/riscv/b-ext.s: Likewise.
	* testsuite/gas/riscv/b-ext-64.d: Likewise.
	* testsuite/gas/riscv/b-ext-64.s: Likewise.
    include/
	* opcode/riscv-opc.h: Added MASK/MATCH/DECLARE_INSN for Zbs.
	* opcode/riscv.h (riscv_insn_class): Added INSN_CLASS_ZBS.
    opcodes/
	* riscv-opc.c (riscv_supported_std_z_ext): Add zbs.

Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>