RISC-V: Use bexti instruction to do single-bit testing

Current RISC-V port tests bit masks with `andi` instruction. But for those mask values not in the range of `simm12` (`andi`
only accepts sign-extended 12-bit immediate [1]), we need an extra temp register (t0 as default for `andi`) to store the mask value [2].
Since we now support Zbs extension of Bit-Manipulation, we have a more convenient way to test power-of-two bit
masks with the single instruction `bexti` [3] without any temp register.

1. https://github.com/riscv/riscv-isa-manual/blob/f6b8d5c7d2dcd935b48689a337c8f5bc2be4b5e5/src/rv32.tex#L519-L521
2. https://github.com/openjdk/jdk/blob/ce6e7461dc5ac56459a79e75d5de76929d1be0a3/src/hotspot/cpu/riscv/macroAssembler_riscv.cpp#L1852-L1860
3. https://github.com/riscv/riscv-bitmanip/blob/main/bitmanip/insns/bexti.adoc