[riscv-port] RFR: 8283364: riscv: Intrinsify countPositives

[Feilong Jiang]

[JDK-8281146](https://bugs.openjdk.java.net/browse/JDK-8281146) replaces `StringCoding.hasNegatives` with `countPositives` and introduces `Op_CountPositives`, which can be intrinsified on riscv port.

New added test test/hotspot/jtreg/compiler/intrinsics/string/TestCountPositives.java passed on QEMU with RVV enabled.

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Commit messages:
 - 8283364: riscv: Intrinsify countPositives

Changes: https://git.openjdk.java.net/riscv-port/pull/67/files
 Webrev: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=67&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283364
  Stats: 45 lines in 3 files changed: 45 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/riscv-port/pull/67.diff
  Fetch: git fetch https://git.openjdk.java.net/riscv-port pull/67/head:pull/67

PR: https://git.openjdk.java.net/riscv-port/pull/67