riscv: Break down -XX:+UseRVB into seperate options for each bitmanip extension

Currently openjdk riscv supports RISC-V bitmanip extension as a bundle while spec provides four individual extensions: Zb[abcs][1].

According to the spec, we need to break down `UseRVB` into two individual options `UseZba` and `UseZbb` to enable or disable Zba and Zbb respectively (openjdk riscv only supports Zba and Zbb for now).

Since multi-letter extensions representation in the ISA bitmap is still not determined[2][3], availability for those extensions could not be queried from HWCAP. Feature detection of Zba and Zbb was removed temporarily.

[1]: https://github.com/riscv/riscv-bitmanip/releases/download/1.0.0/bitmanip-1.0.0-38-g865e7a7.pdf
[2]: http://lists.infradead.org/pipermail/linux-riscv/2021-November/010250.html
[3]: http://lists.infradead.org/pipermail/linux-riscv/2021-November/010252.html