RFR: 8341586: RISC-V: build fail with gcc9

[SendaoYan]

Hi all,
The `src/hotspot/cpu/riscv/icache_riscv.cpp` build fails `riscv_flush_icache.cpp:51:29: error: requested alignment 64 is larger than 16 [-Werror=attributes]` with gcc9. Alignment 16 will be enough.

Addiontional testing:

- [ ] riscv native build with release conf and slowdebug conf
- [ ] jtreg tier1 tests with release build

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Commit messages:
 - 8341586: RISC-V: build fail with gcc9

Changes: https://git.openjdk.org/jdk/pull/21376/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21376&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341586
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/21376.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21376/head:pull/21376

PR: https://git.openjdk.org/jdk/pull/21376