RFR: 8356593: RISC-V: Small improvement to array fill stub

[Anjian-Wen]

When working on [JDK-8351140](https://bugs.openjdk.org/browse/JDK-8351140), I witnessed possible misaligned memory access in array fill stub.
We fill by element for short arrays (< 8 bytes), which assumes a heapword alignment[1]. But that is not guaranteed. This issue could be reproduced by running: `make test TEST="micro:vm.compiler.ArrayFill"`
with `@Param("5") private int size;` on riscv platforms with slow misalgned memory accesses.

[1] https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/riscv/stubGenerator_riscv.cpp#L2141

This fixes this issue by using a small loop to fill the elements for short arrays.
Tier1-2 tested on linux-riscv64 platform. JMH result on P550 SBC for reference:
1. (`@Param("5") private int size;`):

Before:
Benchmark                 (size)  Mode  Cnt    Score   Error  Units
ArrayFill.fillByteArray        5  avgt   12  558.781 ± 1.396  ns/op
ArrayFill.fillIntArray         5  avgt   12   29.346 ± 0.003  ns/op
ArrayFill.fillShortArray       5  avgt   12   30.779 ± 0.004  ns/op
ArrayFill.zeroByteArray        5  avgt   12  559.249 ± 1.909  ns/op
ArrayFill.zeroIntArray         5  avgt   12   29.346 ± 0.002  ns/op
ArrayFill.zeroShortArray       5  avgt   12   30.777 ± 0.006  ns/op

After:
Benchmark                 (size)  Mode  Cnt   Score   Error  Units
ArrayFill.fillByteArray        5  avgt   12  23.977 ± 0.004  ns/op
ArrayFill.fillIntArray         5  avgt   12  29.343 ± 0.004  ns/op
ArrayFill.fillShortArray       5  avgt   12  30.776 ± 0.005  ns/op
ArrayFill.zeroByteArray        5  avgt   12  23.977 ± 0.002  ns/op
ArrayFill.zeroIntArray         5  avgt   12  29.345 ± 0.005  ns/op
ArrayFill.zeroShortArray       5  avgt   12  30.776 ± 0.004  ns/op

2. (`@Param("3") private int size;`):

Before:
Benchmark                 (size)  Mode  Cnt    Score   Error  Units
ArrayFill.fillByteArray        3  avgt   12  428.923 ± 0.409  ns/op
ArrayFill.fillIntArray         3  avgt   12   28.629 ± 0.005  ns/op
ArrayFill.fillShortArray       3  avgt   12  558.872 ± 2.641  ns/op
ArrayFill.zeroByteArray        3  avgt   12  429.744 ± 2.049  ns/op
ArrayFill.zeroIntArray         3  avgt   12   28.628 ± 0.002  ns/op
ArrayFill.zeroShortArray       3  avgt   12  557.682 ± 1.661  ns/op

After:
Benchmark                 (size)  Mode  Cnt   Score   Error  Units
ArrayFill.fillByteArray        3  avgt   12  21.471 ± 0.002  ns/op
ArrayFill.fillIntArray         3  avgt   12  28.631 ± 0.003  ns/op
ArrayFill.fillShortArray       3  avgt   12  20.436 ± 0.288  ns/op
ArrayFill.zeroByteArray        3  avgt   12  21.472 ± 0.002  ns/op
ArrayFill.zeroIntArray         3  avgt   12  28.633 ± 0.003  ns/op
ArrayFill.zeroShortArray       3  avgt   12  20.203 ± 0.137  ns/op

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

Commit messages:
 - RISC-V: Small improvement to array fill stub

Changes: https://git.openjdk.org/jdk/pull/25135/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25135&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8356593
  Stats: 63 lines in 1 file changed: 37 ins; 21 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/25135.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25135/head:pull/25135

PR: https://git.openjdk.org/jdk/pull/25135