RFR: 8309254: Implement fast-path for ASCII-compatible CharsetEncoders on RISC-V

[Dingli Zhang]

[JDK-8274242](https://bugs.openjdk.org/browse/JDK-8274242) propose to extend the x86 ISO-8859-1 encoding intrinsic to work 
also for ASCII-compatible encodings, which helps speeding up various 
CharsetEncoders. Implementing a similar intrinsic should be considered on 
RISC-V as well.

The instruct log with -XX:+PrintOptoAssembly output looks like:

06e +   Encode ISO array R12, R11, R13 -> R10 # KILL R12, R11, R13, R7, V0-V3


## Testing:
qemu w/ UseRVV:
- [x] Tier1 tests (release)
- [x] Tier2 tests (release)
- [x] Tier3 tests (release)
- [x] test/hotspot/jtreg/compiler/intrinsics/string/TestEncodeIntrinsics.java

-------------

Commit messages:
 - 8309254: Implement fast-path for ASCII-compatible CharsetEncoders on RISC-V

Changes: https://git.openjdk.org/jdk/pull/14256/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14256&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309254
  Stats: 48 lines in 4 files changed: 30 ins; 0 del; 18 mod
  Patch: https://git.openjdk.org/jdk/pull/14256.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14256/head:pull/14256

PR: https://git.openjdk.org/jdk/pull/14256