RISC-V: Support aliases for Zbs instructions

Add aliases for the non-immediate mnemonics of b{set,clr,inv,ext} to
yencode the respective immediate insn b{set,clr,inv,ext}i when the
second source operand is an immediate.

2021-01-11  Philipp Tomsich  <philipp.tomsich@vrull.eu>

    gas/
	* testsuite/gas/riscv/b-ext.d: Add tests.
	* testsuite/gas/riscv/b-ext.s: Likewise.
	* testsuite/gas/riscv/b-ext-64.d: Likewise.
	* testsuite/gas/riscv/b-ext-64.s: Likewise.
    opcodes/
        * riscv-opc.c (riscv_opcodes): Add aliases for Zbs.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>