RFR: JDK-8283865: riscv: Break down -XX:+UseRVB into seperate options for each bitmanip extension

[Feilong Jiang]

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.

 Linux RISCV64 release hotspot/jdk tier1 tests are  passed on QEMU with following options:
- [x] +UseZba && +UseZbb
- [x] +UseZba && -UseZbb
- [x] -UseZba && +UseZbb

[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

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

Changes: https://git.openjdk.java.net/jdk/pull/8032/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8032&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283865
  Stats: 132 lines in 8 files changed: 6 ins; 12 del; 114 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8032.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8032/head:pull/8032

PR: https://git.openjdk.java.net/jdk/pull/8032