[riscv-port] RFR: 8279996: riscv: RVB: Add zeros/population count instructions

[Feilong Jiang]

This PR implements zeros and population count instructions: `clz`/`clzw`/`ctz`/`ctzw`/`cpop`/`cpopw`.

New C2 instructions are covered by the following JTREG tests:
- test/hotspot/jtreg/compiler/codegen/Test6823354.java
- test/hotspot/jtreg/compiler/codegen/Test7100757.java
- test/hotspot/jtreg/compiler/codegen/Test8005033.java

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Commit messages:
 - 8279996: riscv: RVB: Add zeros/population count instructions

Changes: https://git.openjdk.java.net/riscv-port/pull/45/files
 Webrev: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=45&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279996
  Stats: 143 lines in 6 files changed: 116 ins; 5 del; 22 mod
  Patch: https://git.openjdk.java.net/riscv-port/pull/45.diff
  Fetch: git fetch https://git.openjdk.java.net/riscv-port pull/45/head:pull/45

PR: https://git.openjdk.java.net/riscv-port/pull/45